Growth Of Aluminum Nitride Films Research Articles

Structural and piezoelectric properties of AlN thin films grown by pressure gradient sputtering

Aluminum nitride (AlN) thin films were grown by pressure gradient sputtering (PGS) and conventional sputtering methods. A pressure gradient with low pressure at a substrate position was measured only in the PGS method. AlN films highly oriented along the c-axis were grown by both methods, but there were remarkable differences in the surface roughness and the piezoelectric properties. The average surface roughness (Ra) obtained by atomic force microscopy images was much smaller in the AlN grown by the PGS method. The piezoelectric constant (d 33) of the AlN grown by the PGS method was higher than that of the conventional method. These results revealed that the PGS technique has an advantage in the growth of AlN films that are highly c-axis oriented with a single dielectric domain.

Growth Mechanism of Semipolar AlN Layers by HVPE on Hybrid SiC/Si(110) Substrates.

In this work, the growth mechanism of aluminum nitride (AlN) epitaxial films by hydride vapor phase epitaxy (HVPE) on silicon carbide (SiC) epitaxial layers grown on silicon (110) substrates is investigated. The peculiarity of this study is that the SiC layers used for the growth of AlN films are synthesized by the method of coordinated substitution of atoms. In this growth method, a part of the silicon atoms in the silicon substrate is replaced with carbon atoms. As a result of atom substitution, the initially smooth Si(110) surface transforms into a SiC surface covered with octahedron-shaped structures having the SiC(111) and SiC() facets. The SiC(111)/() facets forming the angle of 35.3° with the original Si(110) surface act as “substrates” for further growth of semipolar AlN. The structure and morphology of AlN films are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), reflection high-energy electron diffraction (RHEED) and Raman spectroscopy. It is found that the AlN layers are formed by merged hexagonal microcrystals growing in two directions, and the following relation is approximately satisfied for both crystal orientations: AlN()||Si(110). The full-width at half-maximum (FWHM) of the X-ray rocking curve for the AlN() diffraction peak averaged over the sample area is about 20 arcmin. A theoretical model explaining the presence of two orientations of AlN films on hybrid SiC/Si(110) substrates is proposed, and a method for controlling their orientation is presented.

Quasi-2D Growth of Aluminum Nitride Film on Graphene for Boosting Deep Ultraviolet Light-Emitting Diodes.

Efficient and low‐cost production of high‐quality aluminum nitride (AlN) films during heteroepitaxy is the key for the development of deep ultraviolet light‐emitting diodes (DUV‐LEDs). Here, the quasi‐2D growth of high‐quality AlN film with low strain and low dislocation density on graphene (Gr) is presented and a high‐performance 272 nm DUV‐LED is demonstrated. Guided by first‐principles calculations, it is found that AlN grown on Gr prefers lateral growth both energetically and kinetically, thereby resulting in a Gr‐driven quasi‐2D growth mode. The strong lateral growth mode enables most of dislocations to annihilate each other at the AlN/Gr interface, and therefore the AlN epilayer can quickly coalesce and flatten the nanopatterned sapphire substrate. Based on the high quality and low strain of AlN film grown on Gr, the as‐fabricated 272 nm DUV‐LED shows a 22% enhancement of output power than that with low‐temperature AlN buffer, following a negligible wavelength shift under high current. This facile strategy opens a pathway to drastically improve the performance of DUV‐LEDs.

Nucleation and growth behavior of aluminum nitride film using thermal atomic layer deposition

Aluminum nitride (AlN) film possesses wide band gap energy (~6.2 eV) and a high dielectric constant (~9.2), and is resilient to thermal and chemical stimuli. It also exhibits several functionalities, such as piezoelectricity and pyroelectricity. Therefore, AlN film has been used for electronic and optoelectronic devices and micro-electromechanical systems (MEMSs). Among the various methods of AlN thin film growth, atomic layer deposition (ALD) can control film thickness at the nanoscale. Uniform and conformal film growth is possible at temperatures lower than that of chemical vapor deposition or molecular beam epitaxy. Because the ALD process relies on surface chemical reactions, it shows substrate dependency. To control film uniformity from the beginning, an understanding of nucleation and growth behavior on the substrate is necessary. Therefore, the nature of nucleation and growth behaviors on different substrates is investigated. In this study, AlN films are grown on bare Si and TiN substrates at 295–342 °C by thermal ALD using trimethyl aluminum (TMA) and ammonia. Facile nucleation and linear growth on the TiN substrate, and substrate-inhibited nucleation on the Si substrate, are observed. NH3 pretreatment may enhance the growth rate at the nucleation stage. Therefore, the dissociation of NH3 on the substrate is crucial to making uniform nuclei for the subsequent growth of AlN film.

Heteroepitaxial Growth of High-Quality and Crack-Free AlN Film on Sapphire Substrate with Nanometer-Scale-Thick AlN Nucleation Layer for AlGaN-Based Deep Ultraviolet Light-Emitting Diodes.

High-quality and crack-free aluminum nitride (AlN) film on sapphire substrate is the foundation for high-efficiency aluminum gallium nitride (AlGaN)-based deep ultraviolet light-emitting diodes (DUV LEDs). We reported the growth of high-quality and crack-free AlN film on sapphire substrate with a nanometer-scale-thick AlN nucleation layer (NL). Three kinds of nanometer-scale-thick AlN NLs, including in situ low-temperature AlN (LT-AlN) NL, oxygen-undoped ex situ sputtered AlN NL, and oxygen-doped ex situ sputtered AlN NL, were prepared for epitaxial growth of AlN films on sapphire substrates. The influence of nanoscale AlN NL thickness on the optical transmittance, strain state, surface morphology, and threading dislocation (TD) density of the grown AlN film on sapphire substrate were carefully investigated. The average optical transmittance of AlN film on sapphire substrate with oxygen-doped sputtered AlN NL was higher than that of AlN films on sapphire substrates with LT-AlN NL and oxygen-undoped sputtered AlN NL in the 200–270 nm wavelength region. However, the AlN film on sapphire substrate with oxygen-undoped sputtered AlN NL had the lowest TD density among AlN films on sapphire substrates. The AlN film on sapphire substrate with the optimum thickness of sputtered AlN NL showed weak tensile stress, a crack-free surface, and low TD density. Furthermore, a 270-nm AlGaN-based DUV LED was grown on the high-quality and crack-free AlN film. We believe that our results offer a promising and practical route for obtaining high-quality and crack-free AlN film for DUV LED.

Numerical Study of AlN Growth in MOCVD Heated by Induction

In this paper, the process of growing aluminum nitride(AlN) film in the metal organic chemical vapor deposition(MOCVD) reactor heated by induction was simulated, and several important factors affecting the growth rate were analyzed. The purpose of this study is to explore the relationship between these factors and the growth rate of AlN films, and to provide feasible guidance for the parameter optimization of the growth of AlN film. It was concluded that compared with the pure nitrogen(N2) or hydrogen(H2) as the carrier gas, the growth rate was obviously improved under the conditions of using the mixed carrier gases composed of N2 and H2. In addition, the uniformity of AlN film and the growth rate can be improved with the increase of H2 ratio. It was also found that the growth rate of the film gradually decreased with the increased temperature of the gases introduced into the reactor, but the uniformity increased. Dimer is the main growth precursor of the film. In the parasitic reaction, the trimer has the greatest influence on the film, and the concentration of the polymer is more susceptible to temperature changes.

Influence of Process Parameters and Formation of Highly c-Axis Oriented AlN Thin Films on Mo by Reactive Sputtering

It is challenging to grow aluminum nitride (AlN) in c-axis orientation on a metal electrode, primarily due to lattice mismatch and the difference between the coefficients of thermal expansion. In this work, we investigate and optimize the effects of several process parameters such as plasma power, N2 flow concentration, quality of the bottom electrode, and oxygen content. The optimized deposition parameters are necessary for the growth of highly c-axis textured AlN thin film having a thickness of 300 nm on a Mo/SiO2/Si substrate. To this end, c-axis oriented AlN thin films were sputtered on a molybdenum (Mo) electrode at the low substrate temperature (300°C). The quality of the Mo electrode was optimized in terms of the surface roughness, interfacial energy, and full width at half maximum (FWHM) of x-ray diffraction (XRD) peak for facilitating the c-axis growth of the AIN film. Results indicate that the growth of AlN film in (002) orientation on the Mo electrode strongly depends on plasma power, the optimum value of which is found to be 300 W. Also, an extremely low FWHM of AlN (002) peak 0.62° is achieved at ∼ 38% N2 concentration. The oxygen content is also found to be an influential parameter, with a threshold value of 28% by atomic weight, beyond which amorphous growth of the AIN film is observed. The fabricated Mo-AlN-Mo structure has a dielectric constant of 8.89 and a capacitance value of 42 pF measured across the top electrode area of 400 × 400 μm2. The grown c-axis oriented AlN film can be used in applications such as the piezoelectric energy harvester (PEH) and the thin-film bulk acoustic resonator (FBAR).

Growth and Characterization of Polyimide-Supported AlN Films for Flexible Surface Acoustic Wave Devices

Highly c-axis oriented aluminum nitride (AlN) films, which can be used in flexible surface acoustic wave (SAW) devices, were successfully deposited on polyimide (PI) substrates by direct current reactive magnetron sputtering without heating. The sputtering power, film thickness, and deposition pressure were optimized. The characterization studies show that at the optimized conditions, the deposited AlN films are composed of columnar grains, which penetrate through the entire film thickness (~2 μm) and exhibit an excellent (0002) texture with a full width at half maximum value of the rocking curve equal to 2.96°. The film surface is smooth with a root mean square value of roughness of 3.79 nm. SAW prototype devices with a center frequency of about 520 MHz and a phase velocity of Rayleigh wave of about 4160 m/s were successfully fabricated using the AlN/PI composite structure. The obtained results demonstrate that the highly c-axis oriented AlN films with a smooth surface and low stress can be produced on relatively rough, flexible substrates, and this composite structure can be possibly used in flexible SAW devices.

Growth of aluminum nitride films by plasma-enhanced atomic layer deposition

Aluminum nitride films have been grown by plasma-enhanced atomic layer deposition under self-limiting growth and CVD-like conditions. The films have been characterized by IR spectroscopy, ellipsometry, and Auger exposure spectroscopy. We have examined the influence of the deposition temperature, the reactor purge time after exposure to trimethylaluminum vapor, and the plasma exposure time on the growth rate and composition of the films. Under the deposition conditions studied, the growth rate ranged from 0.1 to 0.26 nm per cycle and the refractive index of the films was 1.52 to 1.98. We obtained films with an aluminum to nitrogen atomic ratio near unity.

Effect of sputtering power on surface characteristics and crystal quality of AlN films deposited by pulsed DC reactive sputtering

Aluminum nitride (AlN) is a promising material for use as a substrate for AlGaN‐based ultraviolet LEDs and piezoelectric devices. Among the sputtering methods, pulsed DC sputtering provides a superior growth rate and dense AlN films because of the promotion of two‐dimensional growth. In this study, the effect of sputtering power on surface characteristics, crystal quality, and residual stress of AlN films deposited by pulsed DC reactive sputtering on a nitrided sapphire substrate was investigated. The homoepitaxial growth of AlN films on this substrate gives high crystal quality compared with the use of other substrates such as sapphire or Si for heteroepitaxial growth. Surface damage on the AlN films increased with an increase in sputtering power, leading to a rough surface. At all sputtering powers, c‐axis oriented AlN films were obtained in this study. From the difference in lattice constant compared with the AlN bulk, the inplane compressive stress of the AlN sputtered films was confirmed as residual stress. Residual stress in the AlN films was mostly caused by a peening effect rather than a thermal expansion coefficient difference.

Growth of AlN Films on Silicon Substrates by Radio Frequency Magnetron Sputtering

Polycrystalline aluminum nitride (AlN) films were deposited on Si (111) substrates by radio frequency (RF) magnetron sputtering method in an nitrogen (N2) + argon (Ar) gas mixture. The effect of the preparation conditions- sputtering pressure (p), sputtering power (w), gas mixture (Ar/N2) and post-deposition annealing treatment -on the properties of AlN films were investigated by means of X-ray diffraction (XRD). Highly c-axis oriented AlN films were obtained with optimized growth parameters: p=0.3Pa, w=400w and Ar/N2=2.

High-Frequency Thin-Film AlN-on-Diamond Lateral–Extensional Resonators

In this paper, low-impedance lateral–extensional microresonators are fabricated on a stack of aluminum nitride (AlN) directly deposited on a polished ultrananocrystalline diamond (UNCD) film. The large acoustic velocity of UNCD is utilized to extend the frequency of such resonators beyond 1 GHz while the frequency-defining features are not reduced excessively. In order to promote the growth of a <jats:formula formulatype="inline"><jats:tex Notation="TeX">$c$</jats:tex> </jats:formula>-plane piezoelectric AlN film, the surface of the UNCD film is polished after deposition. Three different UNCD films with different Young's modulus values were prepared, and frequencies up to two times that of similar devices fabricated on silicon have been achieved. The finite-element analysis is employed to evaluate the effect of various physical parameters on the performance of the thin-film piezoelectric-on-substrate resonators in order to achieve very low motional resistance <jats:formula formulatype="inline"><jats:tex Notation="TeX" >$(R_{m})$</jats:tex></jats:formula>. Several resonators were designed with various lateral dimensions and different numbers of support tethers to evaluate the propositions. The lowest <jats:formula formulatype="inline"><jats:tex Notation="TeX" >$R_{m}$</jats:tex></jats:formula> was measured from a multitethered 29th-order thin-film piezoelectric-on-diamond (TPoD) resonator (22 <jats:formula formulatype="inline"><jats:tex Notation="TeX"> $Omega$</jats:tex></jats:formula>) and <jats:formula formulatype="inline"><jats:tex Notation="TeX">$fcdot Q$</jats:tex></jats:formula> product of <jats:formula formulatype="inline"><jats:tex Notation="TeX">$2.72 ast 10^{12}$</jats:tex></jats:formula> at 888 MHz. The temperature coefficient of frequency of this TPoD resonator is measured to be <jats:formula formulatype="inline"><jats:tex Notation="TeX">$-9.6 hbox{ppm}/^{circ} hbox{C}$</jats:tex></jats:formula>, which is much lower than that of the devices fabricated on silicon. Also, this device can withstand input powers up to <jats:formula formulatype="inline"><jats:tex Notation="TeX">$+$</jats:tex></jats:formula>27 dBm, leading to a delivered power density per unit area of <jats:formula formulatype="inline" ><jats:tex Notation="TeX">$sim!!2.9 muhbox{W}/muhbox{m}^{2}$</jats:tex> </jats:formula>.<jats:formula formulatype="inline"><jats:tex Notation="TeX" >$hfill$</jats:tex></jats:formula>[2012-0099]

Effects of laser power on the growth of polycrystalline AlN films by laser chemical vapor deposition method

Polycrystalline aluminum nitride (AlN) films were prepared by laser chemical vapor deposition method using aluminum acetylacetonate and ammonia as source materials. The effects of deposition conditions on the crystal phase, composition and microstructure were investigated. Polycrystalline AlN films were prepared at a laser power above 100W and a deposition temperature above 803K. The microstructure of AlN film changed from aggregated grains to faceted grains to pyramidal grains with increasing laser power and with decreasing total pressure.

直流反应磁控溅射在氮化的蓝宝石衬底上制备AlN薄膜

Aluminum nitride ( AlN ) films were prepared successfully on sapphire and nitrided sapphire substrates by reactive DC magnetron sputtering. The effect of nitridation of sapphire substrate on the growth of AlN films was studied. The films were characterized by X-ray diffraction ( XRD ) , atomic force microscopy ( AFM ) and optical ab- sorption spectrum. XRD patterns of AlN films exhibited a strong preferential c -axis orientation , and nitridation of sap- phire substrate could improve the crystal quality of AlN films and also decrease the residual stress of films. But AFM results revealed that the grain size distribution of films deposited on nitrided sapphire substrates was not more homoge- nous than that of films deposited on sapphire substrates , and optical absorption results also showed nitridation of sap- phire substrate nearly had no effect on the optical behavior of AlN films.

Numerical investigation of pulsed chemical vapor deposition of aluminum nitride to reduce particle formation

Particles of aluminum nitride (AlN) have been observed to form during epitaxial growth of AlN films by metal organic chemical vapor deposition (MOCVD). Particle formation is undesirable because particles do not contribute to the film growth, and are detrimental to the hydraulic system of the reactor. It is believed that particle formation is triggered by adducts that are formed when the group-III precursor, namely tri-methyl-aluminum (TMAl), and the group-V precursor, namely ammonia (NH 3), come in direct contact in the gas-phase. Thus, one way to eliminate particle formation is to prevent the group-III and the group-V precursors from coming in direct contact at all in the gas-phase. In this article, pulsing of TMAl and NH 3 is numerically investigated as a means to reduce AlN particle formation. The investigations are conducted using computational fluid dynamics (CFD) analysis with the inclusion of detailed chemical reaction mechanisms both in the gas-phase and at the surface. The CFD code is first validated for steady-state (non-pulsed) MOCVD of AlN against published data. Subsequently, it is exercised for pulsed MOCVD with various pulse widths, precursor gas flow rates, wafer temperature, and reactor pressure. It is found that in order to significantly reduce particle formation, the group-III and group-V precursors need to be separated by a carrier gas pulse, and the carrier gas pulse should be at least 5–6 times as long as the precursor gas pulses. The studies also reveal that with the same time-averaged precursor gas flow rates as steady injection (non-pulsed) conditions, pulsed MOCVD can result in higher film growth rates because the precursors are incorporated into the film, rather than being wasted as particles. The improvement in growth rate was noted for both horizontal and vertical reactors, and was found to be most pronounced for intermediate wafer temperature and intermediate reactor pressure.

Growth of preferentially-oriented AlN films on amorphous substrate by pulsed laser deposition

Preferentially-oriented aluminum nitride (AlN) films are grown directly on natively-oxidized Si (100) substrate by pulsed laser deposition (PLD) in nitrogen (N2) environment. The AlN preferential orientation changes from (002) to (100) with increasing N2 pressure. Such different behaviors are discussed in terms of deposition-rate-dependent preferential orientation, kinetic energy of depositing species and confinement of laser plume. Finally, sample deposited at 0.9 Pa is proved to have the highest (002) peak intensity, the lowest FWHM value, the highest deposition rate and a relatively low RMS roughness (1.138 nm), showing the optimal growth condition for c-axis-oriented AlN growth at this N2 pressure.

Influence of molybdenum bottom electrodes on crystal growth of aluminum nitride thin films

The authors have found that molybdenum (Mo) bottom electrodes strongly influence the crystal growth of aluminum nitride (AlN) thin films. AlN films were prepared on Si, Mo/Si and Mo/AlN-interlayer/Si substrates. The crystal orientation and microstructure of AlN films prepared on Si substrates strongly depend on sputtering pressure. On the other hand, the crystal orientation and microstructure of AlN films prepared on Mo/Si and Mo/AlN-interlayer/Si hardly depends on sputtering pressure. The local epitaxial growth of AlN films is observed in an almost random texture, and the crystal orientation of the Mo electrodes strongly influences that of the AlN films.

Competition between tensile and compressive stress mechanisms during Volmer-Weber growth of aluminum nitride films

Stress evolution during molecular-beam epitaxy of AIN films was monitored with in situ curvature measurements. Changes in the growth rate produced large stress variations, with more tensile stress observed at higher growth rates. For example, at a growth temperature of 750°C the instantaneous steady-state stress in films with similar grain sizes varied from −0.15GPa at a growth rate of 90nm∕h, to approximately 1.0GPa at a growth rate of 300nm∕h. To explain these results, we develop a kinetic model of stress evolution that describes both tensile and compressive mechanisms. The tensile component is based on a mechanism which is proposed here as an inherent feature of grain-boundary formation. The compressive component is based on our recent model of atom insertion, driven by the excess chemical potential of surface adatoms that is created by the growth flux. The combined model predicts that the stress is largely governed by the competition between tensile and compressive mechanisms, which can be conveniently described with a single parameter, α. The limiting values α→0 and α→+∞ correspond to previous models of compressive and tensile stresses, respectively.

Nearly amorphous to epitaxial growth of aluminum nitride films

Aluminum nitride (AlN) films were prepared by using reactive magnetron sputtering. Substrate temperature T s , radio frequency power P w , and substrate materials (including silicon, platinum coated silicon and sapphire) were varied as deposition parameters. T s – P w diagrams were worked out to illustrate how the film structure can be controlled to vary over a broad range covering nearly amorphous, polycrystalline, texture and epitaxial structures. Increases in T s and P w have the effects of increasing the thermal energy of the species on the substrate surface, and enhancing the crystallization of the deposits and preferential orientation of grains. Sapphire substrate has better lattice matching with the AlN structure, which further facilitates the epitaxial growth of the AlN structure.

Growth of aluminum nitride films at low temperature

The epitaxial growth of aluminum nitride films on Al 2O 3 (0 0 0 1) by sputtering is achieved using a 30 nm thick buffer layer of aluminum and titanium nitride. Both buffer layers facilitate epitaxial growth. In particular, an epitaxial film can be deposited at room temperature if the buffer layer is Al. Pole figures of X-ray diffraction elucidate the crystallinity of films.