Oriented Aluminum Nitride Research Articles

0001] Oriented Aluminum Nitride One-Dimensional Nanostructures: Synthesis, Structure Evolution, and Electrical Properties

This paper presents a systematic investigation on the controlled synthesis of wurtzite aluminum nitride (AlN) one-dimensional (1D) nanostructures in a chemical vapor deposition (CVD) system using Al and NH(3) as starting materials. By controlling reaction temperature and NH(3) flow, nanostructures with manifold morphologies including nanoneedles, branched nanoneedles, short nanorods, slim nanorods, and nanofences were synthesized with high yield and selectivity. The correlation between experiment parameters and product morphologies was interpreted by a surface diffusion based model. Moreover, electrical properties of a single nanoneedle were studied for the first time, in which typical semiconductor characteristics were observed. Silicon was speculated to incorporate into the AlN nanoneedle from silicon substrates during the synthesis, which served as an n-type donor and was responsible for the observed electrical behavior.

Improvement in crystal orientation of AlN thin films prepared on Mo electrodes using AlN interlayers

Highly c-axis oriented aluminum nitride (AlN) thin films have been prepared on molybdenum (Mo) bottom electrodes using AlN interlayers (AlN-IL), by reactive rf magnetron sputtering. The interlayers were deposited between the Mo electrodes and silicon substrates, such as AlN/Mo/AlN-IL/Si. The crystallinity and crystal orientation of the interlayers depend on the interlayer thickness and strongly influence those of the Mo electrodes and AlN films. From transmission electron microscopy observations and X-ray pole figure measurements, the interlayer, Mo electrode and AlN film consist of columnar grains and exhibit a fiber texture. It has been found that they have the local epitaxial relationship of ( 0 0 0 1 ) [ 2 1 ¯ 1 ¯ 0 ] AlN-IL / / ( 1 1 0 ) [ 1 ¯ 1 1 ] Mo / / ( 0 0 0 1 ) [ 2 1 ¯ 1 ¯ 0 ] AlN . The nucleation process of AlN thin films changes from a fine grain structure to a columnar structure.

The characteristics and residual stress of aluminum nitride films grown by two-stage sputtering of mid-frequency power

The [0 0 2] oriented aluminum nitride has a high surface acoustic wave speed and high mechanic-electron couple coefficient. It is a potential material for manufacturing piezoelectric devices in high frequency application. The AlN films deposited onto silicon substrates were fabricated by two-stage sputtering process with mid-frequency generator. The results showed that the film did not have well [0 0 2] preferred orientation at 1.0 and 1.5 kW, and exhibited a [0 0 2] preferred orientation at 2.0 kW. The adhesion was poor when the film had a high preferred orientation because the substrate was damaged by high energetic atoms bombardment. A two-stage growth method was investigated in order to get high [0 0 2] preferred orientation and good adhesion. A good performance was obtained at the first stage power of 1.5 kW and the second stage power of 2.0 kW. The film showed a tensile stress state when the film was deposited at 1.0 kW. In contrast, the stress state was changed to compressive when the films were grown at 2.0 kW. The two-stage growth could succeed not only to get a high [0 0 2] preferred orientation but also to develop a reducing global stress film.

Microstructure analysis of c-axis oriented aluminum nitride thin films by high-resolution transmission electron microscopy

The microstructure, in particular, the surface and interface regions, of the c-axis orientated AlN films deposited on Si (100) substrates was studied. The films showed an evolutionary columnar growth process. In contrast to the previous reports, high-resolution transmission electron microscopy revealed that the AlN films grew directly on substrates without an amorphous interlayer, despite the large lattice mismatch between AlN and Si. The occurrence of misoriented and/or amorphous top layer suggested a subsurface growth/relaxation process of the AlN films by reactive sputtering.

Preparation of Oriented Aluminum Nitride Thin Films on Polyimide Films and Piezoelectric Response with High Thermal Stability and Flexibility

Abstractc‐Axis oriented aluminum nitride (AlN) thin films are successfully prepared on amorphous polyimide films by radiofrequency magnetron reactive sputtering at room temperature. Structural analysis shows that the AlN films have a wurtzite structure and consist of c‐axis oriented columnar grains about 100 nm wide. The full width at half maximum of the X‐ray diffraction rocking curves and piezoelectric coefficient d33 of the AlN films are 8.3° and 0.56 pC N–1, respectively. The AlN films exhibit a piezoelectric response over a wide temperature range, from –196 to 300 °C, and can measure pressure within a wide range, from pulse waves of hundreds of pascals to 40 MPa. Moreover, the sensitivity of the AlN films increases with the number of times it was folded, suggesting that we can control the sensitivity of the AlN films by changing the geometric form. These results were achieved by a combination of preparing the oriented AlN thin films on polyimide films, and sandwiching the AlN and polymer films between top and bottom electrodes, such as Pt/AlN/polyimide/Pt. They are thin (less than 10 μm), self powered, adaptable to complex contours, and available in a variety of configurations. Although AlN is a piezoelectric ceramic, the AlN films are flexible and excellent in mechanical shock resistance.

Effects of Substrate Temperature and Nitrogen Pressure onGrowth of AlN Films by Pulsed Laser Deposition

Highly oriented aluminium nitride (AlN) films are grown on p-Si (100)substrates by pulsed laser deposition, and their characteristics ofstructure and composition are studied by x-ray diffraction, Fouriertransform infrared spectroscopy and scanning electron microscopy. Theresults show that the deposited films exhibit good crystallineproperties with a sharp x-ray diffraction peak at 2θ = 33.15°corresponding to AlN h⟨100⟩ crystallineorientation. The influences of substrate temperature and ambientnitrogen (N2) pressure on the crystallinity of AlN films areremarkable. At room temperature, when the ambient N2 pressurearises from 5×10−6 Pa to 5 Pa, the crystallinity of the filmbecomes better. When the substrate temperature is 600°C, the film hasthe best crystallinity at 0.05 Pa. Furthermore, the effects of substratetemperature and ambient N2 pressure on the combination of Al-Nbonds and surface morphology of AlN films are also studied.

Flexible piezoelectric pressure sensors using oriented aluminum nitride thin films prepared on polyethylene terephthalate films

We have investigated the high sensitive piezoelectric response of c-axis oriented aluminum nitride (AlN) thin films prepared on polyethylene terephthalate (PET) films. The AlN films were deposited using a radio frequency magnetron sputtering method at temperatures close to room temperature. The c axes of the AlN films were perpendicularly oriented to the PET film surfaces. The sensor consisting of the AlN and PET films is flexible like PET films and the electrical charge is linearly proportional to the stress within a wide range from 0to8.5MPa. The sensor can respond to the frequencies from 0.3 to over 100Hz and measures a clear human pulse wave form by holding the sensor between thumb and middle finger. The resolution of the pulse wave form is comparable to a sphygmomanometer at stress levels of 10kPa. We think that the origin of the high performance of the sensor is the deflection effect, the thin thickness and high elastic modulus of the AlN layer, and the thin thickness and low elastic modulus of the PET film.

Growth of highly c-axis oriented aluminum nitride thin films on β-tantalum bottom electrodes

We have investigated the influence of tantalum (Ta) bottom electrodes on the crystallinity and crystal orientation of aluminum nitride (AlN) thin films. AlN thin films and Ta electrodes were prepared by using rf magnetron sputtering method. The crystal structure of the Ta electrodes was tetragonal (β-Ta, a metastable phase) at room temperature. The crystallinity and orientation of the AlN thin films and Ta electrodes strongly depended on sputtering conditions. Especially, the crystallinity and crystal orientation of the Ta electrodes were influenced by their film thickness and the substrate temperature. When the thickness of the Ta bottom electrodes was 200 nm and the substrate temperature was 100 °C, the AlN thin films indicated high c-axis orientation (the full width at half maximum of rocking curve of 3.9°). The crystal orientation of the AlN film was comparable to that of AlN thin films deposited on face centered cubic (fcc) lattice structure metal, such as Au, Pt and Al, bottom electrodes.

Highly c-axis oriented thin AlN films deposited on gold seed layer for FBAR devices

In this study, highly c-axis oriented aluminum nitride (AlN) films are deposited on gold (Au) seed layer using a rf reactive sputtering method under various rf power and sputtering pressures. The evolution of preferred orientation and surface morphologies of the deposited films was studied by x-ray diffraction, scanning electron microscope, and atomic force microscope. It was found that the AlN films prepared using the higher rf power of 400 W and sputtering pressure of 7 mTorr were shown to have the stronger c-axis orientation and exhibit smoother morphologies. The fabricated film bulk acoustic-wave resonator (FBAR) devices considered here with AlN film thickness are 2.25μm, Au and Al are used as bottom electrode and top electrode with the thickness of 0.1 and 0.18μm, respectively. The effective electromechanical coupling coefficient (keff2), quality factor (Qfx) and resonant frequency of the fabricated FBAR device were about 1.47%, 535 and 2.172 GHz, respectively. Moreover, the effect of the top electrode materials, AlN and Al top electrode film thickness on the frequency response characteristics of FBAR devices is also investigated.

Field emission characteristics of oriented-AlN thin film on tungsten tip

(0 0 2) Oriented-aluminum nitride (AlN) films with different thicknesses were deposited on tungsten (W) tips by using radio frequency magnetron reactive sputtering system. Compared studies of field emission (FE) characteristics were performed between the bare and AlN coated W tips. The results showed that enhanced electron emission could be obtained from oriented-AlN film on W tip. The hysteresis behaviors shown in Current–electric field ( I–E) curves during downward electric field sweeps were observed, and the extent of hysteresis in I–E curves strongly depended on the thickness of the AlN film. The stability measurement of FE current presented that the hysteresis could be attributed to the charging in AlN film as an insulator.

Enhancement of the c-axis texture of aluminum nitride by an inductively coupled plasma reactive sputtering process

Inductively coupled plasma reactive sputtering technique has been applied to grow highly oriented aluminum nitride on Si(111) at high process pressure. An inductive coil of 3 and 1/4 turns was introduced to generate a plasma zone near the substrate holder, which provides additional energy to the radicals inside the plasma zone. The full width at half-maximum of the X-ray rocking curve of AlN(0002) is reduced from 7.90° to 4.05° when the inductive coil power is raised from 0 to 180 W. The enhancement of the c-axis texture of AlN is attributed to the increase of the density of the sputtered Al atoms of high energy.

The dispersion properties of surface acoustic wave devices on AlN/LiNbO 3 film/substrate structure

Highly c-axis oriented aluminum nitride (AlN) films were deposited on z-cut LiNbO 3 substrates by reactive rf magnetron sputtering. The crystalline properties investigated by X-ray diffraction (XRD) revealed that AlN film with (0 0 2) preferred orientation was improved by an increase of the deposition time within the experimental range. However, the surface morphology of AlN film measured by scanning probe microscopy (SPM) showed that the roughness was getting worse with increase of deposition time. Surface acoustic wave (SAW) properties, measured by a network analyzer in the structure consisting of highly c-axis AlN films on z-cut LiNbO 3 substrates, were investigated. The phase velocity (V P) was significantly increased by the increase of h/ λ, where h is the thickness of AlN film and λ is the wavelength. However, the insertion loss (IL) of SAW filters was also increased by the increase of h/ λ. Experimental results on the temperature characteristics of SAW devices are also presented.

Green emission from c-axis oriented AlN nanorods doped with Tb

Green emission has been obtained from c-axis oriented aluminum nitride (AlN) nanorods doped with terbium (Tb) on silicon (111). The nanorods were prepared by reactive radio-frequency magnetron sputtering in argon and nitrogen atmosphere using a target of Al and TbN mixtures, and characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The sharp characteristic emission lines observed in the photoluminescence and cathodoluminescence (CL) spectra at room temperature correspond to the Tb3+ intra-4fn shell transitions. The monochromatic (wavelength=554 nm) CL images with the high spatial resolution directly show the green emission from the well-aligned nanorods.

SAW COM-parameter extraction in AlN/diamond layered structures.

Highly c-axis oriented aluminum nitride (AlN) thin piezoelectric films have been grown on polycrystalline diamond substrates by pulsed direct current (DC) magnetron reactive sputter-deposition. The films were deposited at a substrate temperature below 50 degrees C (room temperature) and had a typical full width half maximum (FWHM) value of the rocking curve of the AlN-002-peak of 2.1 degrees. A variety of one-port surface acoustic wave (SAW) resonators have been designed and fabricated on top of the AlN films. The measurements indicate that various SAW modes are excited. The SAW phase velocities of up to 11.800 m/s have been measured. These results are in agreement with calculated dispersion curves of the AlN/diamond structure. Finally, the coupling of modes parameters have been extracted from S11 measurements using curve fitting for the first SAW mode, which indicate an effective coupling K2 of 0.91% and a Q factor of about 600 at a frequency of 1050 MHz.

High velocity SAW using aluminum nitride film on unpolished nucleation side of free-standing CVD diamond.

High performances surface acoustic wave (SAW) filters based on aluminium nitride (AlN)/diamond layered structure have been fabricated. The C-axis oriented aluminum nitride films with various thicknesses were sputtered on unpolished nucleation side of free-standing polycrystalline chemical vapor deposition (CVD) diamond obtained by silicon substrate etching. Experimental results show that high order modes as well as Rayleigh waves are excited. Experimental results are in good agreement with the theoretical dispersion curves determined by software simulation with Green's function formalism. We demonstrate that high phase velocity first mode wave (so-called Sezawa wave) with high electromechanical coupling coefficient are obtained on AlN/diamond structure. This structure also has a low temperature coefficient of frequency (TCF), and preliminary results suggest that a zero TCF could be expected.

Flexible pulse-wave sensors from oriented aluminum nitride nanocolumns

Flexible pulse-wave sensors were fabricated from density-packed oriented aluminum nitride nanocolumns prepared on aluminum foils. The nanocolumns were prepared by the rf magnetron sputtering method and were perpendicularly oriented to the aluminum foil surfaces. The sensor structure is laminated, and the structure contributes to avoiding unexpected leakage of an electric charge. The resulting sensor thickness is 50 μm. The sensor is flexible like aluminum foil and can respond to frequencies from 0.1 to over 100 Hz. The sensitivity of the sensor to pressure is proportional to the surface area. The sensor sensitively causes reversible charge signals that correlate with the pulse wave form, which contains significant information on arteriosclerosis and cardiopathy of a man sitting on it.

Study of aluminium nitride/freestanding diamond surface acoustic waves filters

Highly oriented aluminium nitride (AlN) has been successfully deposited on silicon substrates and on the back side of unpolished, thick, freestanding, polycrystalline diamond films. Structural and electrical properties of the (0 0 2) oriented AlN films have been investigated. Using optimised AlN and chemical vapour deposited diamond films, high quality surface acoustic wave (SAW) filters were constructed by deposition of aluminium inter-digital transducers. The effects of the AlN thickness on the diamond-based SAW filter properties and the SAW propagation were investigated. A phase velocity of 17.1 km/s was observed, which is, to the best of our knowledge, the highest reported value for diamond. The dependence of the phase velocity ( v) on the AlN thickness was compared to theoretical predictions. SAW filters with rather low insertion loss, high suppression and high electromechanical coefficient could be obtained. We also report on piezoelectric d 33 measurements of AlN films by atomic force microscopy.

The growth of AlN films composed of silkworm-shape grains and the orientation mechanism

The oriented aluminum nitride (AlN) films on Si (100) substrates were prepared by reactive magnetron sputtering. It was found that with a low sputtering pressure, it has a high nucleation rate and the (001)-oriented grains are dominant in films. With a high sputtering pressure, the film is mainly consisted of the (100)- and (110)-oriented grains. In the images of atomic force microscopy, the grains show a silkworm-like shape and their long axis is directed along the c-axis of hexagonal AlN. The mechanism about the orientation and the silkworm shape grains were discussed.

Preparation of Highly Oriented Aluminum Nitride Thin Films on Polycrystalline Substrates by Helicon Plasma Sputtering and Annealing

Highly c‐axis‐oriented aluminum nitride (AlN) thin films were prepared on polycrystalline Si3N4 and SiC substrates by helicon plasma sputtering. The difference in the substrate materials scarcely influenced the crystal structures of the films. The full width at half‐maximum (FWHM) of the X‐ray rocking curves of the films was 3.2°, which is the smallest value for AlN thin films deposited on polycrystalline substrates to our knowledge. Annealing at 800°C in vacuum decreased the FWHM from 3.2° to 2.8°. The structure of the films was densely packed and consisted of many fibrous grains. The films developed c‐axis orientation on the polycrystalline substrates, because the interaction between the films and substrate surfaces was small, and (100) and (110) AlN planes grew preferentially. The films were piezoelectric and generated electrical signals in response to mechanical stress.

Synthesis and Surface Acoustic Wave Property of Aluminum Nitride Thin Films Fabricated on Silicon and Diamond Substrates Using the Sputtering Method

C-axis oriented aluminum nitride (AlN) thin films with a thickness of 1 µm were prepared by reactive DC magnetron sputtering on polycrystalline diamond substrates at a substrate temperature of 623 K. The average surface roughness (Ra) of the AlN thin films was less than 2 nm obtained by locating the diamond substrates at a position of 100 mm from the aluminum target. The full width at half maximum (FWHM) of the rocking curve for the AlN(002) peak determined by X-ray diffraction analysis was about 0.2°. The surface acoustic wave (SAW) structures were completed by the deposition of aluminum electrodes on the as-deposited AlN surfaces. The SAW characteristics of an interdigital transducer (IDT)/AlN/diamond structure were investigated. The phase velocity and coupling coefficient were 10,120 m/s and 0.3%, respectively.