Synthesis of c-axis oriented AlN thin films on different substrates: A review

Highly c-axis oriented AlN thin films have been deposited by reactive sputtering on different substrates. The crystallographic properties of layered film structures consisting of a piezoelectric layer, aluminum nitride (AlN), synthesized on a variety of substrates, have been examined. Aluminum nitride thin films have been deposited by reactive pulsed-DC magnetron sputtering using an aluminum target in an Ar/N₂ gas mixture. The influence of the most critical deposition parameters on the AlN thin film crystallography has been investigated by means of X-ray diffraction (XRD) analysis of the rocking curve Full-Width at Half Maximum (FWHM) of the AlN-(0 0 0 2) peak. The relationship between the substrate, the synthesis parameters and the crystallographic orientation of the AlN thin films is discussed. A guide is provided showing how to optimize these conditions to obtain highly c-axis oriented AlN thin films on substrates of different nature.     

Fabrication and performance analysis of film bulk acoustic wave resonators

In this paper, a radio frequency reactive sputtering deposition technique for piezoelectric aluminum nitride (AlN) thin film formation on a gold (Au) bottom electrode and its successful application in a film bulk acoustic resonator (FBAR) are investigated. The X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) measurements show that the AlN films were deposited onto an Au bottom electrode with highly c-axis-preferred orientation, well-textured columnar structure with a fairly uniform grain size of approximately 83 nm. The roughness is measured at a root-mean square (RMS) value of 5.4 nm and the average peak to valley of each grain column is 46.3 nm. The FBAR consists of an AlN piezoelectric thin film sandwiched between Au electrodes, all of which lie on a thin low-stress silicon nitride which serves as a support membrane on silicon. The performance of FBAR device exhibits a significant of the series quality factor (Q_s), the parallel quality factor (Q_p), the effective electromechanical coupling coefficient (k_eff²), and the bandwidths are 97, 120, 5.1%, and 24 MHz, respectively.     

High quality ZnO films deposited by radio-frequency magnetron sputtering using layer by layer growth method

Three-layered ZnO films were deposited on Si substrates by radio-frequency magnetron sputtering using layer by layer growth method. The Raman scattering confocal analysis confirms that ZnO film quality is improving at increasing the number of ZnO layers at film deposition.Applied method of deposition was used to realize homoepitaxial growth of ZnO films on c-Al₂O₃, Si, SiN_x/Si, glass and ITO/glass substrates. In order to improve the film quality we increased the number of deposition stages up to 5. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmittance measurements were used to testify the quality of grown five-layered ZnO films. XRD results showed that all five-layered ZnO films have (002) texture. The second order diffraction peak (004) on XRD spectra additionally testifies to the high quality of all five-layered ZnO films. SEM results demonstrated that no defects such as cracks and dislocations caused by interruption of deposition ZnO films were observed. Transmittance measurement results showed that ZnO films deposited on transparent substrates have abrupt absorption edge and high optical transmission in the visible region of the spectrum.     

Highly c-axis oriented GaN films grown on free-standing diamond substrates for high-power devices

GaN films with highly c-axis preferred orientation are deposited on free-standing thick diamond films by low temperature electron cyclotron resonance plasma enhanced metal organic chemical vapor deposition (ECR-PEMOCVD). The TMGa and N₂ are applied as precursors of Ga and N, respectively. The quality of as-grown GaN films are systematically investigated as a function of deposition temperature by means of X-ray diffraction (XRD) analysis, Hall Effect measurement (HL), room temperature photoluminescence (PL) and atomic force microscopy (AFM). The results show that the dense and uniformed GaN films with highly c-axis preferred orientation are successfully achieved on free-standing diamond substrates under optimized deposition temperature of 400 °C, and the room temperature PL spectra of the optimized GaN film show a intense ultraviolet near band edge emission and a weak yellow luminescence. The obtained GaN/diamond structure has great potential for the development of high-power semiconductor devices due to its excellent heat dissipation nature.     

AlN films sputtered on iridium electrodes for bulk acoustic wave resonators

We report on the growth of highly c-axis-textured aluminum nitride (AlN) films sputtered on iridium layers for thin film bulk acoustic microresonator applications. Iridium films were evaporated on oxidized silicon covered with Ti or Mo/Ti seed layers to improve crystal quality and adhesion. The crystal quality of the AlN films as a function of the crystal structure of the iridium electrode and its pre-conditioning by Ar⁺ bombardment was analyzed by X-ray diffraction, atomic force microscopy and wet chemical etching. Solidly mounted resonators using a single low-impedance layer of silicon dioxide for acoustic isolation were fabricated for a preliminary assessment of the piezoelectric activity of AlN films and the influence of the AlN/Ir stack on the performance of the devices. The electromechanical coupling factor of the AlN films was derived by fitting the electrical response of the resonators to Mason's physical model. AlN films exhibited very high coupling factors (7.5%) barely dependent on the width of the rocking-curve around the AlN 00·2 reflection. The high acoustic impedance of iridium electrodes provided resonators with quality factors higher than those of similar resonators built on lighter electrodes, such as molybdenum.     

Study on the doping effect of Li-doped ZnO film

Zinc oxide (ZnO) is an excellent piezoelectric material with simple composition. ZnO film is applied to the piezoelectric devices because it has high resistivity and highly oriented direction at c-axis. Structural and electrical properties in ZnO films are influenced by deposition conditions. Lithium-doped ZnO (LZO) films were deposited by RF magnetron sputtering method using Li-doped ZnO ceramic target with various ratios (0 to 10 wt.% LiCl dopant). LZO films revealed high resistivity of above 10⁷ Ω cm with smooth surface when they were deposited with 4% LiCl-doped ZnO target under room temperature. However, their c-axis orientation was worse than the c-axis orientation of pure ZnO films. We have also studied on structural, optical and electrical properties of the ZnO films by XRD, AFM, SEM, XPS, and 4-point probe analyses. We concluded that LZO films were deposited with 4 wt.% LiCl-doped ZnO target and were apposite for piezoelectrical application.     

Influence of incidence angle and distance on the structure of aluminium nitride films prepared by reactive magnetron sputtering

Aluminum nitride (AlN) films were reactively deposited on (100) oriented silicon substrates by reactive radio frequency (RF) magnetron sputtering for different incidence angles and distances between substrate and target.X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to consider the influence of process parameters such as reactive gas flow rate, grazing incidence angle (α), and distance (d) between substrate and target surface on the property of AlN films. XRD results showed that AlN film prepared at a constant distance (d) of 3 cm and an incidence angle of 45° revealed a mixture of AlN (002), (100), and (101) planes, while the film prepared at α = 0° revealed a strong AlN (002) orientation which has a perpendicular growth direction to the substrate surface. AFM results showed that AlN film prepared at α = 0° exhibited more flat surface morphology than that of film prepared at α = 45°.     

Effect of AlN buffer layer on the microstructure and bandgap of AlN films deposited on sapphire substrates by pulsed laser deposition

AlN buffer layer is proposed to improve the growth of AlN films on the sapphire substrate by pulsed laser deposition. The buffer layers were pre-deposited under vacuum for different time, which was aimed to suppress the negative nitridation effect in the initial growth stage, and their effects on the surface morphology, crystal structure and bandgap of AlN films were characterized. It is found that AlN-buffered films exhibit a single (0002) preferred orientation and the crystallinity improves as the pre-deposition time increases from 5 to 20 min. Al-polarity AlN films are obtained at the pre-deposition time of 5 and 10 min, while the polarity inversion from the Al- to N-polarity is observed at 20 min. Based on the analysis of optical transmittance spectra, the bandgap of AlN films decreases with increasing pre-deposition time, which may be resulted from the decrease of axial ratio c/a.     

Characterization of reactively sputtered c-axis orientation (Al, B)N films on diamond

In this research, we demonstrated the viability of oriented AlN layer that incorporated BN to enhance the texturing. Wurtzite (Al, B)N films were deposited on a diamond wafer (diamond film on Si wafer) by a co-sputtering technique. The preferred orientation structure is sensitive to sputtering control factors. The relationship between the microstructures and process conditions were examined with XRD, TEM and AFM analysis. The cross-section TEM images showed that amorphous and randomly aligned structures were produced in the initial sputtering period, but the higher c-axis orientation structure formed as the sputtering time increased. The thickness of the amorphous and randomly aligned layer decreased with increasing sputtering power, nitrogen concentration, substrate temperature and bias voltage. As the thickness of the amorphous and the randomly aligned layer decreased, an (Al,B)N film with higher film quality than AlN was observed.     

Superconducting behavior of high-T c YBa2Cu3O7 thin films with BaO impurity produced by pulsed laser deposition

We have investigated the superconducting behavior of high-T _c YBa₂Cu₃O₇ (YBCO) thin films containing BaO impure phase produced by pulsed laser deposition. The thin films were characterized by the standard four-probe method, X-ray diffraction (XRD), and scanning electron microscopy (SEM). XRD showed that all these thin films contained BaO impurity, with thec-axis normal to the surface of the substrates. The presence of impurity existed from substrate temperatureT _s of 727 to 796°C. When these thin films with BaO impurity were measured under the magnetic fields, it was found that the critical current densityJ _c increased slightly with increase in magnetic fieldB within the range ofB≤500 G, in the case ofB perpendicular to thec-axis of the film.     

Effects of the Ar–N2 sputtering gas mixture on the preferential orientation of sputtered Ru films

We have investigated the influence of N₂ addition to the Ar sputtering gas on the crystal orientation of sputtered Ru films. An rf magnetron sputtering apparatus with a Ru target (99.9%) and a glass substrate heated to 100 °C or 300 °C was used for the deposition. The crystal structure, chemical composition and electrical properties of the resultant films were investigated. X-ray diffraction (XRD) revealed the dominant orientation at 0% N₂ to be the c-axis. With increasing proportion of N₂ in the sputtering gas at a substrate temperature of 100 °C, the intensity of the (002) peak decreased, finally disappearing at 50% N₂. This c-axis-suppressed Ru film sputtered at 50% N₂ was found to contain nitrogen by Auger electron spectroscopy (AES), but by annealing the film in vacuum at 400 °C, the nitrogen in the film was completely removed. The film orientation remained the same as before annealing. Thus, we have demonstrated a new method for depositing Ru films with a controlled preferential orientation of either c-axis oriented or c-axis suppressed.     

On the properties of AlN thin films grown by low temperature reactive r.f. sputtering

In this paper we report on the growth of c-axis-oriented AlN thin films by low temperature reactive r.f. sputtering and the results of their examination and analysis in a variety of ways. In addition to using other substrates, we fabricated c-axis- oriented AlN films on aluminium film substrates. A hexagonal column structure was observed in the morphology of replicas of the natural surfaces of films. As well as the dielectric constant ε, the resistivity ϱ, the breakdown field E_p and the refractive index n, curves of ε versus frequency f, ε versus temperature T and conductivity σ versus T were measured. The IR absorption spectrum of an AlN film sputtered at low temperature coincides with that of an AlN film sputtered onto a high temperature substrate and of a bulk crystal, which remains unchanged after annealing. The band gap E_g = 5.9–6.0 eV, which remains unchanged after annealing in N₂ at 900°C. Theoretical calculation of the dispersion curve of surface acoustic waves by an AlN/glass structure shows that the curve is very level in the range hk = 0.2–0.6. The capacitance-voltage curve of an Al/AlN/Si structure is given in this paper.     

Preparation of Epitaxial MgO Films Deposited by RF Magnetic Sputtering on the IBAD-MgO Substrate

We deposited epi-MgO films on the textured ion beam assisted deposition (IBAD)-MgO substrates by RF magnetic sputtering at different substrate temperatures (600–850 °C), RF powers (110–224 W) and oxygen partial pressures (19.5–58.6 mTorr). The microstructure and surface morphology of epi-MgO films were characterized by X-ray diffraction (XRD) and atom force microscope (AFM). It was found that epi-MgO films with c-axis orientation could be easily fabricated for broad parameter ranges. But the in-plane full width half maximum (FWHM) of the epi-MgO film was dependent on the parameters, and the epi-MgO film with the smallest FWHM value of 5.22° was obtained at the optimum parameters. What’s more, the GdBa₂Cu₃O₇ films deposited on the epi-MgO/IBAD-MgO substrate by RF magnetic sputtering showed c-axis orientation.     

Dependence of Zr content on electrical properties of Bi3.15Nd0.85Ti3-xZrxO12 thin films synthesized by chemical solution deposition (CSD)

The Bi_3.15Nd_0.85Ti_3-xZr_xO₁₂ (BNTZ) thin films with Zr content (x = 0, 0.05, 0. 1, 0.15, and 0.2) were prepared on Pt/Ti/SiO₂/Si (100) substrates by chemical solution deposition (CSD) technique. The crystal structures of BNTZ films were analyzed by X-ray diffraction (XRD). The effects of Zr contents on the ferroelectric, dielectric properties, and leakage current of BNTZ films were thoroughly investigated. The XRD results demonstrated that all the films possessed a single phase bismuth-layered structure and exhibited the highly preferred (117) orientation. Among these films, the film with Zr content x = 0.1 held the maximum remanent polarization (2P_r) of 50.21 μC/cm² and a low coercive field (2E_c) of 210 kV/cm.     

Investigation of ZnO films on Si<111> substrate grown by low energy O+ assisted pulse laser deposited technology

Zinc oxide thin films (ZnO) with different thickness were prepared on Si (111) substrates using low energy O⁺ assisted pulse laser deposition (PLD). The structural and morphological properties of the films were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements, respectively. The quality of ZnO films was also examined by using Rutherford backscattering spectroscopy/ion channeling (RBS/C) techniques. XRD showed that there was only one sharp diffraction peak at 2θ = 34.3° with the full width at the half maximum (FWHM) of around 0.34° for two ZnO samples, which also indicated that ZnO thin films had a good c-axis preferred orientation. Results of Rutherford backscattering and ion channeling clearly indicated that the Zn:O ratio in zinc oxide thin film approached to unity and the ZnO thin film grown by low energy O⁺ assisted pulse laser deposition had a polycrystalline structure. In the case of ZnO film fabricated by low energy O⁺ assisted pulse laser deposited under identical experimental conditions except growth time, AFM analysis has shown that the root mean square (RMS) roughness (2.37 nm) of thinner ZnO film (35 nm) was far below that (13.45 nm) of the thicker ZnO film (72 nm).     

Influence of the seed layer on structural and electro-acoustic properties of sputter-deposited AlN resonators

The growth of high-quality AlN films has been studied by reactive sputtering onto Mo electrodes with Ni, Ti, and TiW seed layers and subsequent integration into thin film bulk acoustic wave resonators. The crystalline structure and morphology of the Mo and c-axis oriented AlN films were found to vary strongly with seed layer material and thickness. The smoothest Mo electrodes were obtained on thin Ti films. Reactive sputtering of AlN on top of these optimized electrodes resulted in a dense columnar grain structure with a well-aligned (002) crystal orientation and good electro-acoustic properties, including an effective coupling coefficient of 6.89% and quality factor above 1000.     

Growth of highly oriented LiTaO3 thin film on Si with amorphous SiO2 buffer layer by pulsed laser deposition

High-quality single-phase, c-axis textured LiTaO₃ thin films have been deposited on Si(100) substrate with amorphous SiO₂ buffer layer for optic waveguide application by pulsed laser deposition under optimized conditions of 30 Pa oxygen pressure and 650 °C. The amorphous SiO₂ buffer layer with a thickness of 100 nm was coated on the Si(100) by thermal oxidation at 1000 °C. Li-enriched LiTaO₃ ceramic target was used during the deposition. In order to study the influence of oxygen pressure on the orientation, crystallinity and morphology, different oxygen pressures (10 Pa, 20 Pa, 30 Pa and 40 Pa) were used. X-ray diffraction (XRD) results showed that LiTaO₃ thin films exhibited highly c-axis orientation under 30 Pa. It was observed by scanning electron microscopy (SEM) that the as-grown film in the optimal conditions was characterized by a dense and homogeneous surface without cracks, and the average grain size was in the order of 25 nm.     

Physical properties of CuCrO2 films prepared by pulsed laser deposition

High-quality CuCrO₂ films were prepared by pulsed laser deposition (PLD). The film deposited with the pulse energy density (PED) of 2 mJ/cm² is highly c-axis oriented. The refractive index of the CuCrO₂ films is about 1.29 obtained by transmission spectra of the films, which implies that the CuCrO₂ film will be a potential antireflection coating in visible light. The films prepared with different PEDs show different conduction mechanism, which suggested the different band structure between these CuCrO₂ films.     

Structural and optical characterization of c-axis textured BaTiO<Subscript>3</Subscript> thin films on MgO fabricated by RF magnetron sputtering

In this paper, BaTiO₃ thin films were prepared by RF magnetron sputtering on MgO substrates and their properties such as the crystal structure and optical waveguide properties were investigated. The optimum deposition parameters, such as substrate temperature, deposition pressure, gas flow ratio, the RF power and the after annealing temperature, were obtained in order to get the best BaTiO₃ film quality. The XRD results show that highly c-axis textured BaTiO₃ thin films were successfully grown on MgO substrate. Films obtained under the optimum deposition parameters, substrate temperature of 650°C, RF power of 50 W, deposition pressure 18 mTorr and gas flow ratio O₂/(Ar+ O₂) of 15% namely, reaches a full width at half maximum intensity (FWHM) of BaTiO₃ (002) XRD peak of 0.25°. The FWHM of BaTiO₃ (002) XRD peak was further reduced to 0.24° via post-treatment with furnace annealing (at 800°C for 2 h) which indicates the film crystal quality is further improved. The bright and sharp TE modes measured by m-line spectroscopy of the BaTiO₃ film were observed indicating its possible application in optical waveguide.     

Effect of deposition angle on fiber axis tilt in sputtered aluminum nitride and pure metal films

We show that the fiber axis orientation in sputtered aluminum nitride (AlN) responds strongly to deposition angle as compared with pure metal films. Fiber axis tilt was measured in films deposited at angles from 0° to 68° from the substrate normal. For pure metal films of Al and Ag, the strong (111) texture has a fiber axis tilt of < 10° from the substrate normal. For pure Nb films, the strong (110) texture also has a tilt of < 10°. In contrast, for films of the compound AlN, the distinct c-axis texture responds strongly to the deposition direction, with the fiber axis tilt almost following the deposition angle.