A1N Films Research Articles

AlN Films Deposited by LP-MOCVD Atomic Layer Deposition at Lower Temperatures Using DMEAA and Ammonia

ABSTRACTWe have investigated the deposition of AlN thin films on Si(100), Al2O3(0001), and Al2O3(0112) substrates at lower temperatures (523–723 K) using a novel aluminum source, dimethylethylamine:alane (DMEAA), with ammonia as a nitrogen source in a low-pressure MOCVD atomic layer growth process. At reactor pressures of 25 and 50 Torr a four-step sequence of reactant flow steps separated by flush steps was cycled. We observed a tendency toward a self-limiting growth rate as the DMEAA step flow time was increased. The deposition uniformity was observed to be dependent on temperature and non-uniform deposition occurred at higher temperatures. The microstructure and crystalline orientation were examined using x-ray diffraction and crystalline A1N films were deposited at temperatures as low as 573 K. Crystallite size decreased with substrate temperature and at 523 K. amorphous films were deposited. At T > 650 K preferentially oriented crystalline films were deposited with orientations of Si(100)//AIN(0001), Al2O3(0001)//AIN(0001), Al2O3(0112)//AIN(1120).

Optical properties of sputter-deposited aluminium nitride films on silicon

The optical properties of thin sputter-deposited A1N films on Si were measured. It is shown that they depend on the exact preparation conditions. In the visible range, the refractive index n of A1N is fitted by n = c 1 exp(- λ/ c 2) + c 3, where c 1 = 2.09 and c 2 = 131.9 nm are constant and c 3 ≈ 1.8 –2.5 depending on the preparation conditions. It is also shown that the chromaticity coordinates of A1N on Si vary quasi-periodically with the product of the refractive index and its thickness.

Polycrystalline Grain Structure of Sputtered Aluminum Nitride Films

ABSTRACTReactively-sputtered, polycrystalline thin film aluminum nitride (AlN) is an attractive material for use in acoustic wave devices, for which it requires a strong preferred orientation, similar to that found in epitaxial films. This investigation evaluated the grain structure including preferred orientation, grain size, and surface morphology of sputtered A1N films. The characterization techniques utilized included x-ray diffraction (XRD), secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The results revealed two types of grain structure: 1) a single-grain columnar structure that is perfectly oriented in the [001] direction throughout the entire film thickness and 2) a multiple-grain columnar structure that possesses a strong [001] orientation at the bottom of the film and a tilted [001] combined with other orientations at the top of the film. Strong correlations between orientation and surface morphology, oxygen content, and grain size were observed, namely higher degrees of c-axis orientation correlated with lower mean surface roughness values, reduced oxygen concentration, and narrower grains.

Epitaxial Growth of Aluminum Nitride on Sapphire and Silicon

ABSTRACTThe microstructural characteristics of wide band gap semiconductor, hexagonal A1N thin films on Si(100), (111), and sapphire (0001) and (10ī2) were studied by transmission electron microscopy (TEM) and x-ray diffraction. The films were grown by MOCVD from TMA1 + NH3 + N2 gas mixtures. Different degrees of film crystallinity were observed for films grown on α-A12O3 and Si substrates in different orientations. The epitaxial growth of high quality single crystalline A1N film on (0001) α-Al2O3 was demonstrated with a dislocation density of about 2*10 10cm−2 . The films on Si(111) and Si(100) substrates were textured with the c-axis of A1N being perpendicular to the substrate surface.

Influence of plasma exposure in the preparation of A1N films by facing-target sputtering

A1N films were prepared by facing-target sputtering in Ar(50%) + N 2(50%) at gas pressures below 1.33 Pa. On X-ray diffraction signal measurement, the (00.2) signal intensity decreased and its full width at half-maximum increased with decreasing gas pressure, the values for films prepared in Ar + N 2 being larger than those in N 2. The spacing between adjacent (00.2) planes was also extended at lower Ar + N 2 gas pressures. These results were induced in film growth under plasma exposure of the A1N film. This was also confirmed by preparing A1N films in a system where a grounded net was inserted between the substrate and plasma to diminish the ion bombardment of the film. Furthermore, it was shown that A1N films prepared by facing-target sputtering were c-axis-oriented films regardless of the partial pressure of N 2.

Properties of reactively d.c.-magnetron-sputtered A1N thin films

In this work the dependence of the properties of reactively d.c.-magnetron-sputtered A1N films on their deposition parameters was investigated. In this set-up, argon was used as the sputtering gas and nitrogen as the reactive gas. The argon-to-nitrogen ratios varied between 1 to 3 and 3 to 1, at three different total gas flows: 50 sccm, 100 sccm and 200 sccm. The d.c. power was kept constant at 1 kW. All films showed a polycrystalline hexagonal structure; the refractive index at 550 nm of the series grown at 50 sccm was scattered around 2.12, for the series grown at 100 sccm around 2.05, and for the series grown at 200 sccm around 1.97. Localized gap states appeared in films grown at 50 sccm with an Ar-to-N 2 ratio of 3 to 1. The films grown at flows of 100 sccm and 200 sccm showed tensile stresses (up to 1 GPa). The films grown at 50 sccm showed a transition from compressive ( −1.3 GPa) to tensile stress (0.3 GPa) if the Ar-to-N 2 ratio was increased to more than 2 to 1.

Real Time Stress Measurements During Growth of Aluminum Nitride on SI(111) and SI(001)

ABSTRACTWe have performed real time measurements of intrinsic stresses during growth by reactive dc magnetron sputtering of aluminum nitride (AlN) thin films on silicon substrates in an UHV growth chamber. An experimental setup based on laser beam reflection is constructed such that substrate curvature as well as film thickness can be continuously monitored as growth proceeds. On Si(111) substrates, stress measurements were carried out during growth of both polycrystalline and epitaxial A1N films as a function of deposition pressure. This is the first such comparative study to our knowledge for the AlN/Si system. Our room temperature measurements on polycrystalline films corroborates previous post-growth measurements. Our high temperature measurements provide evidence of large intrinsic stresses and negligible stress relaxation during epitaxial growth of AlN on Si(111). We further compared stress behavior during both room temperature and high temperature growth of AlN films on Si(111) and Si(001) substrates. Our observations indicate while intrinsic stresses during room temperature growth can be compressive or tensile depending on plasma conditions, it is tensile during late stage growth at high temperatures.

Vaporazation and condensation of metals in a flowing gas with high velocity

Ultrafine Ag particles were prepared by evaporation and condensation in a forced flowing He gas with a high velocity up to 30m/s. From electron microscopic observation, followed by statistical analysis of particle size distributions for specimens collected at various positions in the gas stream, the condensation process occurred in two regions having different morphology; film formation region and particle growth region. The particle growth region is divided into three subregions having different mechanisms of particle growth: absorption growth, coalescence growth, and intermediate or combined growth. By applying a low-temperature plasma generated by microwave, A1N films and particles having wurtzite-type structure were formed. This technique is widely applicable to produce pure compound films and particles.

Epitaxial growth of A1N film by low-pressure MOCVD in gas-beam-flow reactor

We suggested “gas-beam-flow” condition and “gas-beam-flow” type reactors for epitaxial A1N growth using trimethylaluminum and ammonia. By flow visualization experiments, two gas flows of the sheath and the source were found to be laminar at atmospheric and at low pressures in the “gas-beam-flow” condition. The source gas flowed without touching the reactor wall. Using the “gas-beam-flow” type reactor, we succeeded in growing epitaxial A1N films at low temperature of 690 °C with a superior surface morphology.

Crystalline orientation control for aluminum nitride films prepared by ion-beam-assisted technology

Aluminum nitride films were prepared by evaporation of aluminum and simultaneous bombardment of nitrogen ions in the low-energy region 200–1000 eV (IVD method) on substrates of fused silica and on Si (100), (111) wafers. The ion incident angle was changed from 0° to 45° from the normal to the substrate surface, and the relation between the direction of ion incidence, ion beam energies, and crystalline orientation of A1N films was studied. X-ray diffraction patterns indicated that each film had a hexagonal structure and the direction of the (002) axis was parallel to the ion beam direction. Every film showed the same tendency irrespective of the substrate used. In addition, the dielectric constants of the A1N films were measured by means of an impedance analyzer and ellipsometry. The relation between the direction of crystalline orientation and the dielectric constant was also studied.

Low temperature growth of A1n and Al2O3 films by the simultaneous use of a microwave ion source and an ionized cluster beam system

Abstract We have prepared aluminium nitride (A1N) and oxide (Al2O3) films at a substrate temperature of 100°C by the simultaneous use of a microwave ion source and an ionized cluster beam system. For A1N, transparent and amorphous films with high packing density were obtained. Both A1N and Al2O3 films were chemically and thermally stable. Furthermore, the Al2O3 films could be made polycrystalline by increasing the incident energy of the oxygen ion beam and the acceleration voltage of ionized aluminium clusters. The electrical resistivity of the A1N and Al2O3 films was higher than 5 × 10 13 Ω cm and the breakdown voltage was larger than 3 × 106V cm−1. The chemical reaction between aluminium and reactive gas (i.e. nitrogen and oxygen) could be promoted by ionizing the reactive gas. Also, the ionization of aluminium clusters could improve the quality of the films as a result of combination with the reactive gas ions even at a low substrate temperature.

Surface Studies of Aluminum Mitride Thik Films

ABSTRACTThe nature and the composition of the surface and near surface region of A1N films plays a significant role in determining their chemical and thermal stability. A1N thin films were deposited by reactive RF magnetron sputtering of Al in argon and nitrogen atmosphere. Transmission and scanning electron microscopy indicated that these films have a dense columnar microstructure with columns oriented along the c-axis of the A1N crystallites. X-ray photo-electron spectroscopy (XPS) revealed a graded oxygen-rich surface layer about 5–10 nm thick which is formed when the film is exposed to the atmosphere. This oxide is limited to the surface and does not grow even when the film is annealed at 600°C for 10 hours in dry oxygen. It is expected that this passivating oxygen rich surface layer will, to a large extent, determine the oxidation resistance of the fresh underlying AIN. A surface chemistry model is proposed for this protection behavior.

Fabrication Techniques and Properties of Piezoelectric Thin Films of ZnO and AlN

Piezoelectric films of ZnO and AlN have been fabricated by sputtering. The film is assessed to be of good quality by means of X-ray diffraction, scanning electron microscopy, resistivity and electromechanical measurements. The ZnO/glass and ZnO/crystal composites have been used as SAW devices. A c-axis oriented AlN film has been grown on a glass or metal film substrate at substrate temperature as low as 20∼500°C. An epitaxial ZnO and A1N film has been grown onto a basal plane sapphire substrate. A SAW propagated along c-axis of the ZnO films on the (011̄2) planes of sapphire will be available as a high frequency filter device above 1 GHz.

Piezoelectric thin films for saw applications

Abstract Piezoelectric films of ZnO, AIN and KLN have been fabricated by rf planar magnetron sputtering. The sputtering deposition rate (≃10μm/h) of the c-axis oriented ZnO films has been increased by one order of magnitude by using planar magnetron sputtering compared to a usual rf sputtering. The film is evaluated as of good quality using X-ray diffraction, scanning electron microscopy, reflection electron diffraction, optical measurement, and electromechanical measurement. The ZnO/glass and ZnO/metal composites have been successfully used as SAW TV-IF filters and electronic watch oscillators. An epitaxial AIN film has been grown onto a basal plane sapphire substrate and also a c-axis oriented AIN film has been grown on a glass or metal-film substrate at substrate temperatures as low as 20–500°C. Because of this low substrate temperature, AIN films have become available for bulk wave transducers, SAW transducers with the IDT's between A1N films and substrates, and SAW transducers on low melting point ma...

Deposition of A1N thin films by magnetron reactive sputtering

A1N films were deposited onto silicon substrates in d.c. planar magnetron system by means of reactive sputtering of aluminium in an atmosphere of nitrogen. The influence of the deposition conditions and of post-deposition annealing on the properties of the insulator and the A1NSi interface were investigated.

Electrical properties of sputtered A1N films and interface analyses by Auger electron spectroscopy

A1N films were deposited at room temperature by reactive sputtering. Silicon surfaces, A1N films and A1NSi interfaces were analysed by Auger electron spectroscopy. The effects of ion and chemical silicon substrate etchings and the effects of A1N post-deposition annealings on the aluminum, nitrogen, silicon, oxygen and carbon depth profiles, especially in the interface region, are discussed. Leakage currents and C-V characteristics of the metal-A1NSi structures were determined. The dielectric constant of A1N was 11.3±0.5 at 1 MHz and the density of fixed interface charges varied between +2.5×10 11 and -5×10 10 cm −2. Oxygen and positive fixed charges induced at the A1NSi interface by the native SiO 2 layer on the silicon substrate were removed by annealing at 650°C. This study showed that the electrical characteristics were improved when the native SiO 2 was not etched before the deposition of A1N.

Oriented growth of A1N films on oriented A1 films

It has been found that oriented A1N films can be grown at relatively low substrate temperature on thin oriented aluminum films. A1N films have been prepared by dc glow discharge in pure N 2 gas in the temperature range from room temperature to about 400 °C. Aluminum films, which had been deposited on NaCl single crystals at several temperatures and annealed at the growth temperature of A1N in vacuum, have been used as substrates. The structure and orientation of the films have been studied by transmission electron diffraction.

Epitaxial growth and piezoelectric properties of A1N, GaN, and GaAs on sapphire or spinel

Heteroepitaxial films of the III-V compounds, A1N, GaN and GaAs have been grown on insulating substrates by reactions involving Group III metal-organic compounds and Group V hydrides. The films were examined with respect to crystallography, surface topography, uniformity, residual strain, and electrical and acoustic properties with emphasis on those orientations which are of particular interest to surface acoustic wave (SAW) device applications. Aluminum nitride films up to 10 µm in thickness were grown on 1″ diameter sapphire substrates with a 5% to 10% thickness variation. The films, though characterized as single crystal by x-ray means, exhibited a grain-like structure and considerable surface faceting. The residual strain in the films depends on the crystallographic direction and increases substantially with film thickness. These films exhibit useful surface acoustic properties. Epitaxial GaN films are more easily prepared than A1N films but by contrast are semiconducting unless “doped” with Zn or Li during the growth process. Films of this material are similar crystallographically to A1N and preliminary results show that they exhibit piezoelectric properties. The lack of published data on the acoustic properties of GaN films is probably due to the difficulty in compensating the films to provide insulating layers in device structures. Preliminary results obtained on GaAs epitaxial layers are discussed briefly because of the semiconducting properties of this material.