Polycrystalline AlN Research Articles

Sublimation growth of AlN bulk crystals in Ta crucibles

AlN single crystals of 0.5 in diameter and up to 10–12 mm long have been grown by sublimation/recondensation in pre-treated tantalum crucibles. Growth of 45 mm diameter and 4 mm long polycrystalline AlN boules has also been demonstrated. After high-temperature pre-treatment in a carbon-containing atmosphere, the tantalum crucibles can be used for 300–400 h of AlN sublimation growth at 2200–2300 °C, without Ta impurities or additional C impurities in concentrations higher than 100 ppm appearing in the crystals. Both self-seeded growth of polycrystalline AlN on the crucible lid and seeded growth of single-crystal AlN on (0 0 0 1) SiC plates and AlN/SiC templates are demonstrated. X-ray diffractometry and topography of the grown crystals show a block structure with the characteristic block size >200 nm and the scatter of FWHMs of ω -scans in the range of 60–750 arcsec. Test slicing and polishing of the crystals and test MBE growth of AlGaN/AlN structures on the obtained AlN substrates have been successfully performed.

Structural and optical characterization of AlN films grown by pulsed laser deposition

AlN thin films were prepared on p-type Si(1 0 0) substrates heated at 800 °C by pulsed laser ablation of AlN targets using an UV KrF* ( λ = 248 nm, τ FWHM ≤ 10 ns) excimer laser. We report herewith new results in depositing AlN films from AlN targets and their characterization by X-ray diffraction (XRD), along with Fourier transform infrared (FTIR) investigations in reflection and spectroscopic ellipsometry data. The X-ray investigations confirm the formation of polycrystalline AlN films. We observed the complete absence of the Al line in the XRD spectra. The gradual decomposition of the AlN target in the zones beneath and around the crater, induced by nanosecond multipulse laser irradiation, was compensated by a low-pressure N 2 flux (0.1–10 Pa) during deposition. The reflection IR spectra display features characteristic to LO phonons in AlN. Ellipsometric measurements evidenced a refractive index of 2.00 and an extinction coefficient of 0.0001 for AlN films with a thickness of about 100 nm.

Diffusion and activation of Zn implanted into InP:S

The aim of this work is to compare carrier concentration, diffusion coefficient and shape of the zinc atomic profile in InP layers treated under different annealing conditions. Zn implanted InP:S (4×10 18 at/cm 3—active dopant) bulk samples were investigated. Samples were implanted to a fluence of 1×10 16 cm −2 at an energy of 150 keV and then coated with a 100 nm thick AlN polycrystalline film, using sputter deposition followed by annealing at temperatures in the range 500–900 °C for various different conditions. After removal of the AlN films the extent of diffusion of the implanted species was characterized using the SIMS (Secondary Ion Mass Spectrometry) technique whilst carrier concentration was evaluated by C– V profiling. The depth profiles of in-diffused zinc strongly indicated that the diffusion coefficient D was concentration-dependent. The diffusivity of Zn was found largest when InP was annealed with phosphorus overpressure combined with the AlN cap; in case of annealing at 700 °C a Zn concentration of 2×10 19 cm −3 was achieved. The diffusion range under phosphorus overpressure was almost twice that in vacuum and this is interpreted in terms of the generation of additional vacancies in the In sub-lattice. C– V measurements showed clearly that application of protective AlN together with phosphorus overpressure causes sulphur donors compensation and results in acceptor concentration of 1×10 18 cm −3 and consequently, in the formation of a well-defined p–n junction. The activation efficiency of Zn atoms can be estimated to be about 30%, without additional heat treatment. The Boltzmann–Matano analysis was employed to evaluate the concentration-dependent diffusion coefficient of Zn in InP.

Seeded growth of bulk AlN crystals and grain evolution in polycrystalline AlN boules

Large AlN crystals were grown by powder sublimation in a nitrogen atmosphere at low supersaturation and growth rates of 0.1–0.3 mm/h. The starting deposition surface was a sintered TaC disc. An appropriate adjustment of the system pressure and source-seed temperature gradient during the early stages of growth allowed epitaxial re-growth on AlN seeds that had been exposed to air. Single-crystalline AlN grains of 1 cm in size were achieved through multiple sublimation growth runs conducted at P = 500 Torr and growth temperatures of 2050–2150 °C. Elemental analysis of impurities in the grown AlN boules confirmed low oxygen contamination levels of ∼10 19/cm 3. No discontinuities were introduced in the structural defect distribution in the individual single-crystalline grains by the multiple re-growth steps. Absence of preferred growth directions of grains suggest the epitaxial re-growth process is suitable for seeded single-crystal growth in any orientation.

Growth of AlN crystals on AlN/SiC seeds by AlN powder sublimation in nitrogen atmosphere

AlN single crystals were grown on AlN/SiC seeds by sublimation of AlN powder in TaC crucibles in a nitrogen atmosphere. The seeds were produced by metallorganic chemical vapor deposition (MOCVD) of AlN on SiC crystals. The influence of growth temperature, growth time and source-to-seed distance on the crystallinity and the crystal growth rate were investigated. Crystals were grown in an RF heated sublimation reactor at growth temperatures ranging from 1800-2000°C, at a pressure of 600 Torr, nitrogen flow-rate of 100 sccm and source-to-seed distances of 10 and 35 mm. At 1870°C and a source-to-seed distance of 35 mm, isolated crystals were observed with few instances of coalescence. At 1930°C, a source-to-seed distance of 10 mm and longer growth times (~30 hrs), crystal coalescence was achieved. Above 1930°C, the decomposition of SiC was evidently affecting the growth morphology and resulted in growth of polycrystalline AlN. After an initial nucleation period, the observed growth rates (10-30 µm/hr) were in close agreement with predictions of a growth model that assumed gas-phase diffusion controlled growth. Optical and electron microscope observations revealed step-flow growth, while X-ray diffraction results showed the single crystal nature of the grown material. Single crystalline AlN was grown over surface areas of 200-300 mm2 and was transparent and essentially colorless.

Preparation of AlN:Mn films by metalorganic chemical vapor deposition for thin film electroluminescent devices

AlN:Mn films with Mn concentrations ranging from 2 × 1018 to 1 × 1021 cm−3 have been prepared by metalorganic chemical vapor deposition using bismethylcyclopentadienyl-manganese as a Mn source. The films grown at 1050 °C were polycrystals, whereas those grown at substrate temperatures (Ts) lower than 800 °C were mixture of polycrystalline and amorphous AlN, with the ratio of the latter phase increasing with decreasing Ts. The films showed red–orange photoluminescence based on the transitions of d-electrons in Mn4+ ions at room temperature. The intensity of the Mn-related emission increased with decreasing Ts. The Ts dependence of PL was discussed in conection with the structual properties of the films. The electroluminescence (EL) properties were also investigated by fabricating thin film EL devices on glass substrates. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural, morphological and acoustic properties of AlN thick films sputtered on Si(001) and Si(111) substrates at low temperature

Polycrystalline AlN thick films were deposited on Si(001) and Si(111) substrates by reactive radio frequency sputtering technique at low temperature. The structure and the morphology of the films were investigated by X-ray diffraction, scanning electron microscopy and atomic force microscopy techniques. These measurements showed that the AlN films were highly c-axis oriented, with low surface roughness. The surface acoustic wave (SAW) properties of the films were investigated: a mean value of 3.8×10 −12 C/N was estimated for the piezoelectric strain constant d 33; the phase velocities of SAWs propagating in polycrystalline AlN/(001)[110]Si and AlN/(111)[1–10]Si structures, for different film thicknesses, were calculated and found to be in good agreement with the theoretical velocities evaluated for SAWs propagating in single crystal AlN/Si structures.

X-ray photoelectron spectroscopy of polycrystalline AlN surface exposed to the reactive environment of XeF 2

X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and scanning electron microscopy (SEM) measurements of polycrystalline AlN surfaces exposed to XeF 2 vapor at 1.8×10 −4 Torr at sample temperatures ranging from T s=300 to 920 K are reported. No change in the chemical composition and morphology of the AlN surface exposed at T s=300 K was observed from a virgin AlN surface. The AlN surface exposed at T s=700 K is partially fluorinated, forming an AlF 3 layer, whereas those exposed at T s=750 and 800 K are found to be completely covered with an AlF 3 layer although no change was observed in morphology. Above T s=850 K, however, partially fluorinated AlF x ( x=1, 2) species and exposed AlN surface area were observed but no AlF 3-passivated layer was detected, probably because the AlF 3 formed is desorbed as soon as it forms. Therefore, it is concluded that, above T s=850 K, the fast corrosion reaction of AlN by XeF 2 proceeds and thus the AlN surface is strongly etched.

Quantitative basis for the rocking-curve measurement of preferred orientation in polycrystalline thin films

A quantitative basis for the rocking-curve measurement of the preferred orientation in polycrystalline thin films is presented. Gaussian functions are used for modeling the density distribution of the normals to the crystal plane around the normal to the specimen surface. An intensity formula for the rocking curve is derived from the kinematical theory applied to the case of asymmetric Bragg reflection. The density distribution is determined by the least-squares fit of a theoretical rocking curve to the observed curve, and a volume fraction of crystallites, whose normals to the crystal plane are present within a defined angular range, can be obtained from it. AlN and Au polycrystalline thin films were used for testing the present procedure. Parameter values of the model function, refined using both synchrotron radiation and laboratory X-rays, agree well with each other within the experimental errors although these intensity data sets were collected under different experimental conditions in instrumentation and wavelength. A distribution of depth-dependent preferred orientation in the AlN thin film was revealed by using double-layer and multiple-layer models. A very small degree of preferred orientation in Au thin films could also be measured. Parallel-beam optics and integrated intensities instead of peak height intensities are important for reliable rocking curve measurement.

Influence of crystal properties on the absorption IR spectra of polycrystalline AlN thin films

In this paper Fourier transform infrared (FTIR) transmission spectroscopy is presented as a technique suitable for the routine morphological study of polycrystalline AlN thin films. We have compared FTIR spectra and X-ray diffraction (XRD) patterns of wurtzite polycrystalline AlN films deposited on Si(001) by RF reactive sputtering of an Al target in an N 2/Ar mixture. The preferred orientation and crystal quality of the films were investigated by XRD. The FTIR spectra showed absorption bands due to vibrational modes of AlN bonds, in particular, the A1(TO) mode at 612 cm −1 and the E1(TO) mode at 672 cm −1. The ratio of integrated areas of A1(TO) and E1(TO) absorption bands is related to the preferred orientation of the films, which ranges from films with purely (00.2)-oriented grains to films with grains tilted by approximately 60° to the surface normal. Energy shifts of the IR absorption modes were observed in relation to biaxial residual stress present in the films. As residual stress varied from tensile to compressive, E1(TO) peak energy increased, while A1(TO) peak energy decreased. It is concluded that FTIR transmission spectroscopy has the potential to be used as a routine characterization technique for in-line process control of polycrystalline AlN thin films.

Inductively coupled plasma nitriding of aluminium

Substrates of aluminium alloy 2011 were plasma nitrided using an inductively coupled plasma source. The plasma nitriding parameters of temperature, length of nitriding and negative dc bias of the substrates were varied in order to optimise the plasma nitriding process. Substrates were characterised by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). All nitriding was carried out at a rf power level of 100 W and pressure of 6.5×10 −3 Torr N 2 . AlN was detected by XPS on all plasma nitrided Al substrates but the relative contribution of AlN to the total Al photoelectron signal was not significant (<15%) for Al substrates treated at low temperatures (50 and 450 °C) and low negative dc bias (100 and 150 V, respectively). AlN was not detected by XRD in these instances. Significant formation of AlN was observed both by XPS and XRD after nitriding for 180 min at a temperature of 575 °C whilst the substrate was biased with a negative dc voltage of 400 V. AlN formed at 575 °C both with and without prior Ar sputtering of the naturally occurring aluminium oxide over layer giving rise to an AlN contribution to the total Al photoelectron signal of 49 and 21%, respectively. XRD analysis confirmed the presence of a surface polycrystalline AlN layer. Both AlN and Al 2 O 3 were also observed by XPS on the surface of the nitrided Al substrates. SEM revealed a nodular morphology of the nitrided surfaces which were black in colour and exhibited a 2–3 μm thick surface layer. Nitriding of an Al substrate at 485 °C with dc bias of −400 V gave rise to a 35% AlN contribution to the total Al photoelectron signal. However, no AlN XRD pattern was observed. This indicates formation of a thin AlN layer only.

Mass transport mechanisms during excimer laser nitriding of aluminum

Surface layers of aluminum nitride were formed by irradiating pure aluminum substrates in nitrogen atmosphere with a pulsed excimer laser. The beam was focused on the sample placed inside a chamber filled with nitrogen gas. The irradiation was carried out at various laser fluences, nitrogen gas pressures, and numbers of pulses in order to investigate the influence of each parameter on the nitrogen incorporation and the mass transport mechanisms. X-ray diffraction showed the formation of polycrystalline AlN phase with the wurtzite structure, and the analysis of the nitrogen depth profiles by means of resonant nuclear reaction Analysis revealed a monotonic increase of the nitrogen concentration with the ambient gas pressure and the number of laser shots. It has been found that the laser fluence directly determines the temperature of the substrate and strongly changes the transport mechanism. The thermal simulations and the experimental evidence show that for fluences higher than 3 J/cm 2 the temperature of the substrate exceeds 2900 K. This value is higher than the dissociation temperature (∼2400 K) and close to the melting point (∼3070 K) of AlN, which can therefore dissociate or melt. The atomic nitrogen can rapidly diffuse to greater depths in the liquid Al matrix or it can degas (outgas) through the surface of the sample, leading to the formation of rather homogeneous concentration profiles. For fluences lower than 3 J/cm 2 the temperature of the substrate is not sufficient to destroy the nitride phase and the AIN grains can move inside the molten Al. In this case, the material transport can be attributed to Brownian motion and thermophoretic drift, which in turn are correlated with the chemical and thermal gradient, respectively.

Chemical vapour deposition of polycrystalline AlN films from AlCl 3–NH 3 mixtures: II — surface morphology and mechanisms of preferential orientation at low-pressure

Polycrystalline AlN films were prepared from AlCl 3/NH 3 mixtures by chemical vapour deposition (CVD) under low-pressure conditions. Various surface morphologies and preferred orientations were observed, depending on the operating parameters selected. The ammonia flow rate was found to be particularly important in determining the preferred orientation of the deposited layers. The crystal habits were found to be dominated by slow-growing pyramidal and pedion facets. A mechanism based on evolutionary selection was proposed to account for the preferred orientations produced in the layers, but it was also suggested that the orientation of the tiny crystals formed during the early stages of growth is not completely random. Finally, the numerical model developed in our previous article was used to derive a correlation between the fraction of surface covered by ammonia and the final preferred orientation of the deposited layers.

Oxidation behavior of AlN substrate at low temperature

The oxidation onset and the kinetics of polycrystalline AlN substrates were studied by measuring the weight percent of oxygen in the surface layer and the surface roughness with energy dispersive X-ray spectroscopy (EDX) and atomic force microscopy (AFM), respectively. The oxidation started in the temperature range 800–900 °C and the entire surface of the AlN substrate was covered with an Al2O3 oxide layer below 1100 °C. The oxidation kinetics followed a linear rate law below 1000 °C and a parabolic rate law above 1100 °C. Above 1100 °C, the surface roughness increased abruptly by the irregular shape of overgrown oxide, which might enhance the adhesion of metal to the AlN surface in a metallization process. With an increase of the oxidation temperature above 1200 °C, the oxide layer split during cooling due to the thermal expansion mismatch between the AlN matrix and the Al2O3 oxide layer.

Structural and electroacoustic studies of AlN thin films during low temperature radio frequency sputter deposition

AlN is a material used in a wide variety of applications such as electroacoustic devices, blue diodes, IR windows, thermal conductors, metal–insulator–semiconductor structures, integrated circuit packaging, etc. In this work thin piezoelectric AlN polycrystalline films have been grown on Si and SiO2 using rf magnetron sputter deposition in an Ar/N2 gas mixture. The structural properties of the films have been optimized by varying the deposition parameters, such as process pressure, gas mixture, substrate temperature, discharge power, etc. [K. Tominaga et al., Jpn. J. Appl. Phys., Part 1 35, 4972 (1996); H. Okana et al., ibid. 31, 3446 (1992); K. Kazuya, T. Hanabusa, and K. Tominaga, Thin Solid Films 281-282, 340 (1996)]. It was found that the best film texture was obtained for a particular set of parameters, namely process pressure of 4 mTorr, substrate temperature 350 °C, discharge power 350 W, and a gas mixture of 25% Ar and 75% N2. The films as examined by x-ray diffraction exhibited a columnar structure with a strong (001) texture, and a full width at half maximum (FWHM) rocking curve of 1.6°. Atomic force microscopy measurements indicated a surface roughness with a rms value of 8 Å. Classical nonapodized transversal surface acoustic wave filters operating at a frequency of 534 MHz were fabricated to characterize the electroacoustic properties of the films. The measurements indicated a coupling coefficient of 0.37% and a phase velocity of 4900 m/s. Further, thin epitaxial films were grown on (001)α-Al2O3 (sapphire) under the same deposition conditions except the substrate temperature. The films exhibited a (001)AlN//(001)α-Al2O3 plane orientation with a (002) rocking curve FWHM value of about 0.4°, showing a relatively good alignment of the c axis. The in-plane orientation was [110]AlN//[120]α-Al2O3 corresponding to a rotation of the AlN film of 30° with respect to the (001)α-Al2O3 surface. Cross-sectional transmission electron microscopy studies indicated a population of both thread and edge dislocations with decreasing concentrations with film thickness.

MBE grown monocrystalline GaAs films on polycrystalline AlN thick films for power device applications

MBE grown monocrystalline GaAs films on polycrystalline AlN thick films for power device applications

Preparation of AlN polycrystalline plate from aluminum plate

AlN was produced by nitriding electrolytically etched aluminum plate. It was found that the plate shape did not change. (100) Oriented aluminum plate (18×50×0.1 mm in size, 99.99% in purity) was suitable for the conservation of plate shape. Electrolytic etching and preheating at 600°C in N2 flow are essential for the complete nitriding. Although nitriding temperature (1100–1400°C) was much higher than the melting point of aluminum, the plate was completely nitrided without the destruction of shape. The surface film of aluminum oxide, which was formed by a trace amount of oxygen in N2 gas, conserved the plate shape during nitriding. The relative density of product was increased to 91% by sintering at 1800°C for 2 h.

Orientation of aluminum nitride films grown with hyperthermal molecular beams

Unskimmed, pulsed hyperthermal molecular beams of trimethylaluminum (TMA) and ammonia were used to grow films of aluminum nitride. The crystal structure of the films was determined by X-ray diffraction (XRD). AlN was deposited on Si(1 0 0) and AlN–Si. The AlN–Si samples consist of 1 μm layer of MOCVD-grown polycrystalline AlN on Si(1 0 0) substrates. The resulting film orientation on Si(1 0 0) is with the AlN( 1 0 1 ̄ 0 ) plane parallel to the substrate plane. On AlN–Si substrates, the AlN( 1 0 1 ̄ 0 ) plane is still prevalent. The effect of varying the precursors’ azimuthal angle of incidence, α, was also investigated. AlN was grown on Si(1 0 0) at α=0, 30, 60°. For α>0, the AlN( 1 0 1 ̄ 0 ) and AlN( 1 0 1 ̄ 1 ) planes are present. At α=60°, weak X-ray diffraction peaks indicate the films have short-range order. For the α examined, the average growth rate is approximately 0.35 μm/h. AES was used for chemical analysis of films.

Effect of extended annealing cycles on the thermal conductivity of AlN/Y 2O 3 ceramics

The effect of extended annealing cycles (up to 50 h at 1800°C) on the thermal conductivity of polycrystalline AlN, doped with 5 wt% Y 2O 3, has been studied. The microstructural evolution upon annealing has also been characterized in detail, using quantitative scanning electron microscopy (SEM) observation and energy dispersive X-ray analysis (EDX). As-sintered AlN/Y 2O 3 composites typically contained a dilute yttrium aluminate secondary phase well distributed and completely wetting the AlN grains. Upon annealing, the AlN matrix grains isotropically grew, while the grain-boundary yttrium aluminate phase tended to segregate to triple grain junctions. This segregation process produced a collapse of the grain-boundary film thickness, thus resulting in a completely different AlN microstructure dispersed with isolated yttrium aluminate grains. Equilibrium of the microstructural morphology was achieved after annealing times in the interval 5–10 h. As a consequence of microstructural changes, the thermal conductivity of the annealed AlN polycrystal exceeded that of the as-sintered material. A discussion is given about the variation of thermal properties in terms of both segregation to the triple-grain junctions of the intergranular Y 2O 3-phase and grain-growth of the bulk AlN grains.

Synthesis of highly oriented piezoelectric AlN films by reactive sputter deposition

Nucleation and growth of polycrystalline AlN films on thermal and chemical vapor deposited oxide have been studied during rf reactive sputter deposition. The influence of the growth conditions, namely deposition pressure, rf power, Ar/N2 ratio, and substrate temperature, on film properties has been systematically studied. The properties of interest are crystallinity, degree of orientation, crystallite size, surface roughness, stress, piezoelectric coupling, acoustic velocity, and others. The films have been analyzed with Rutherford backscattering spectroscopy, electron spectroscopy for chemical analysis, x-ray diffraction (XRD), ellipsometry, scanning electron microscopy, atomic force microscopy, stress measurements, etc. It is found that these properties are sensitive functions of all deposition parameters and that there exist optimal deposition conditions under which films of high quality are obtained. The films at optimal conditions were analyzed with the following results: full width half maximum (FWHM) XRD 0.216°, FWHM rocking curve 1.62°, crystallite size 38 nm, optical index 2.15, surface roughness 31 Å, and stress 400 MPa. Further, to study the electroacoustic properties of the films surface acoustic wave filters were fabricated operating at 534 MHz. The thin film structure consists of AlN/SiO2/Si. The electrodes of the interdigital transducers were made of Al. Examination of the frequency response indicated an acoustic velocity of 4900 m/s and a moderate coupling coefficient.