Epitaxial AlN Films Research Articles

Epitaxial growth of cubic AlN films on SrTiO 3(1 0 0) substrates by pulsed laser deposition

Cubic AlN thin films have been fabricated on SrTiO 3(1 0 0) substrates by pulsed laser deposition (PLD) at different substrate temperatures and ambient nitrogen pressures. The microstructure and surface morphology of the deposited films were characterized by reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectrometry (XPS). AlN films fabricated at 450 °C had polycrystalline structures. Epitaxial cubic AlN films were obtained when the deposition temperature was increased to 650 °C. The epitaxial relationship of cubic AlN film on SrTiO 3 substrate was AlN[1 0 0]∥SrTiO 3[1 0 0] and AlN(2 0 0)∥SrTiO 3(1 0 0). The degradation of the film crystalline quality was found if the growth temperature was further increased to 800 °C. AFM investigation revealed that the surface morphology of AlN films strongly depended on N 2 partial pressure. The root mean square (RMS) roughness values for AlN films deposited at 650 °C and at 10 Pa N 2 were about 0.674 nm. In addition, XPS results demonstrated that no oxide phase existed in epitaxial AlN films. Thus, epitaxial cubic AlN films could be fabricated on STO substrates under the optional deposition conditions by PLD.

Epitaxial growth mechanisms of AlN on SiC substrates at room temperature

Epitaxial AlN films have been grown on atomically flat 6H-SiC (0001) and 4H-SiC (0001) substrates at room temperature (RT), and their growth mechanisms have been investigated. It has been revealed that the growth of AlN at RT proceeds in a layer-by-layer mode from the initial stage. During the growth, AlN nucei have an equilateral triangular shape probably due to the anisotropy in growth rates. The triangular AlN nuclei at the adjacent terraces formed on the 6H-SiC substrates are rotated by 60°, whereas the triangular AlN nuclei on the 4H-SiC were formed in the same direction on all of the terraces. This phenomenon can be explained by the difference in the arrangement of the atoms for the topmost layers.

Epitaxial growth of AlN films on single-crystalline Ta substrates

We have demonstrated the first epitaxial growth of AlN films on single-crystalline Ta substrates by the use of a low-temperature growth technique based on pulsed laser deposition (PLD). Although previous AlN films grown on Ta(100) and (111) substrates have exhibited quite poor crystallinity, an epitaxial AlN(0001) film with an in-plane epitaxial relationship of AlN[112¯0]//Ta[001] has been obtained on a Ta(110) substrate at a growth temperature of 450 °C. We found that the full-width at half-maximum values for the crystal orientation distribution in the tilt and twist directions of the AlN film were 0.37° and 0.41°, respectively. Grazing-incidence X-ray reflection (GIXR) and X-ray photoelectron spectroscopy (XPS) measurements have revealed that the AlN/Ta heterointerface is quite abrupt, and that its abruptness remains unchanged even after annealing at 1000 °C.

Relation between Al vacancies and deep emission bands in AlN epitaxial films grown by NH3-source molecular beam epitaxy

Intensity ratios of characteristic deep cathodoluminescence (CL) bands at 4.6, 3.8, and 3.1eV to the near-band-edge emissions at 11K of AlN epilayers grown by NH3-source molecular beam epitaxy were correlated with the change in the S parameter of positron annihilation measurement, which represents the concentration or size of Al vacancies (VAl). Since the relative intensities of 3.1 and 3.8eV bands increased remarkably with lowering supply ratio of NH3 to Al (V/III ratio) and growth temperature (Tg), they were assigned to originate from VAl-O complexes. The VAl concentration could be decreased by adjusting V/III ratio and Tg, resulting in observation of fine excitonic features in the CL spectra. From the energy separation between the ground and first excited states, the binding energy of A exciton was determined to be 48meV.

Fabrication of particle free epitaxal AlN thin films by reactive PLD combined with an electron beam and a rotating crucible

An AlN epitaxial film without projections or spiral growth features was successfully fabricated on a (0 0 0 1) sapphire substrate by reactive laser ablation of a liquid Al target in NH 3 using a 248 nm laser. The liquid Al target was prepared with an electron beam in a rotating crucible. The surface of the rotating liquid Al target was always smooth. Spiral growth features were greatly suppressed. AlN films have a surface roughness less than 0.3 nm. The X-ray rocking curve's narrowest FWHM was 180 arcsec, which is nearly identical to that recently reported in films grown by MOCVD or reactive MBE.

Investigation of AlN films grown by molecular beam epitaxy on vicinal Si(111) as templates for GaN quantum dots

The use of AlN epitaxial films deposited on vicinal Si(111) as templates for the growth of GaN quantum dots is investigated by transmission electron microscopy and atomic force microscopy. It is found that the substrate vicinality induces both a slight tilt of the AlN (0001) direction with respect to the [111] direction and a step bunching mechanism. As a consequence, a dislocation dragging behavior is observed giving rise to dislocation-free areas well suited for the nucleation of GaN quantum dots.

PVT Growth at High Temperatures

Physical vapor transport (PVT) growth method was used to grow bulk crystals of AlN and thick epitaxial SiC films on on-axis Si-face 4H-SiC(0001) substrates from the powder source materials. Both bulk and thick film crystals were characterized by scanning electron microscopy (SEM), which showed a surface morphology did not have any observable growth defects or cracking. We observed nice hexagonal growth morphology a typical characteristic of diffusion-limited growth. River-patterns free from micro bubbles and micro voids were observed on the as grown surface. An XRD rocking curve measured the FWHM as 51.0 arc seconds and a reciprocal space map showed a singular intense region that is attributed to the high quality homoepitaxial SiC film. A similar value was observed for small AlN crystals also.

A study of dislocations in AlN and GaN films grown on sapphire substrates

The dislocations in epitaxial AlN film directly grown on (0 0 0 1) sapphire have been investigated by transmission electron microscope and X-ray diffraction (XRD), including direct comparison with conventional GaN film grown on sapphire substrates. Unlike GaN, the threading dislocations (TDs) of AlN are almost entirely of pure-edge character, and the screw dislocation density is much less than that of GaN film, leading to a very narrow full-width-at-half-maximum of ∼70 arcsec in (0 0 0 2) XRD of AlN film. The origin of screw TDs in GaN is found to be primarily related to the highly faulted layer at the interface region, which is introduced by using a low temperature (LT) nucleation layer. The high temperature AlN growth without any LT initial layer leads to the dramatic reduction of screw dislocations in AlN. Simultaneously, the AlN growth is dominated by two-dimensional step-flow mode while the island growth that occurs for GaN film on sapphire is suppressed. Moreover, we obtained a very low dark current density of 1.1×10 −9 A/cm 2 at a 5 V bias on AlN metal-semiconductor-metal diodes, which indicates a lower defect density and good material quality of our AlN thin film.

Investigation of layered structure SAW devices fabricated using low temperature grown AlN thin film on GaN/sapphire

Epitaxial AlN films have been grown on GaN/sapphire using helicon sputtering at 300 degrees C. The surface acoustic wave (SAW) filters fabricated on AlN/GaN/sapphire exhibit more superior characteristics than those made on GaN/sapphire. This composite structure of AlN on GaN may bring about the development of high-frequency components, which integrate and use their semiconducting, optoelectronic, and piezoelectric properties.

Near-bandedge cathodoluminescence of an AlN homoepitaxial film

Cathodoluminescence experiments were performed on a high-quality AlN epitaxial film grown by organometallic vapor phase epitaxy on a large single crystal AlN substrate. The low-temperature near-bandedge spectra clearly show six very narrow lines. The thermal quenching behavior of these emission lines provides insight on how to assign them to free and bound exciton recombination processes. The binding energy for the free-exciton-A in AlN was found to be nearly twice that in GaN. The observation of the free-exciton-A first excited state permitted us to estimate its reduced effective mass and, by using recent reported values for the hole effective mass in Mg-doped AlN, the electron effective mass in AlN has been deduced.

High‐quality AlN epitaxial films on (0001)‐faced sapphire and 6H‐SiC substrate

This paper presents crystal qualities of high-quality AlN epitaxial films on (0001)-faced sapphire and6H-SiC substrates. The AlN epitaxial films are grown using a low-pressure metal organic vapor phase epitaxy (LP-MOVPE) method. 0.5–1 μm-thick AlN films without any cracks are realized on both substrates. Both of AlN films are found to have similar crystal qualities in spite of large difference in in-plane lattice mismatch between AlN and each substrate. The AlN films have an atomically flat surface with clear atomic steps. Results of X-ray rocking curve (XRC) measurement indicate that both of the AlN films have small tilted mosaicity, however relatively large twisted mosaicity. Dislocation density of the AlN films in its surface region is approximately as low as 1 × 1010 cm−2 and most of dislocations consist of edge-type dislocations. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Generalized infrared ellipsometry study of thin epitaxial AlN layers with complex strain behavior

The effect of film thickness on the strain and structural properties of thin epitaxial AlN films has been investigated, and a sub-layer model of the degree of strain and related defects for all films is suggested. The vibrational properties of the films have been studied by generalized infrared spectroscopic ellipsometry. The proposed sub-layer model has been successfully applied to the analysis of the ellipsometry data trough model calculations of the infrared dielectric function.

AlN/Diamond np-junctions

The first aluminum nitride/diamond heterojunction light-emitting diode is realized by plasma-induced molecular beam epitaxy. The pn heterojunction consists of a silicon doped AlN (n-type) epitaxial film on (100) naturally boron-doped (p-type) diamond substrate. The diode shows excellent rectifying properties with intense light emission in the spectral range of approximately 2.7 and 4.8 eV. Heterojunctions manufactured from AlGaN alloys and diamond hold great promise for new applications in optoelectronics as well as in high frequency and high power electronics.

Two-step Process for the Metalorganic Chemical Vapor Deposition Growth of High Quality AlN Films on Sapphire

Epitaxial AlN films were deposited on (0001) sapphire using a new metalorganic chemical vapor deposition process in which optimal substrate nitridation is combined with modulated ammonia flow and a growth pressure reduction from 150 torr to 40 torr after the first stage of growth. A significant improvement in the full width half maximum (FWHM) of the rocking curves was obtained. The best layers had FWHM of 330 arc sec/650 arc sec for the (0002)/(1012) reflections, respectively.

High-quality AlN epitaxial films grown by MOVPE method.

High-quality AlN epitaxial films grown by MOVPE method.

Growth of Crack-Free and High-Quality AlGaN with High Al Content Using Epitaxial AlN (0001) Films on Sapphire

High-quality AlGaN with high Al content was grown on high-quality epitaxial AlN film on sapphire (0001) by low-pressure metal organic vapour phase epitaxy (LP-MOVPE). The obtained AlxGa1—xN (0.2 < x < 0.8) thicker than 0.9 μm had atomically smooth surface without any cracks. Full width at half maximum (FWHM) values of X-ray rocking curve (XRC) from the AlGaN were less than 200 arcsec for (0002) diffraction. It is considered that in-plane compressive stress in the AlGaN due to larger in-plane lattice constant of the AlGaN than that of the underlying AlN plays an important role in restraining generation of cracks.

Piezoresponse force microscopy for piezoelectric measurements of III-nitride materials

Piezoelectric constants and polarity distributions of epitaxial AlN and GaN thin films are investigated by piezoresponse force microscopy (PFM). The magnitude of the effective longitudinal piezoelectric constant d 33 is determined to be 3±1 and 2±1 pm/V for wurtzite AlN and GaN/AlN layers grown by organo-metallic vapor phase epitaxy on SiC substrates, respectively. Simultaneous imaging of surface morphology as well as the phase and magnitude of the piezoelectric response is performed by PFM on a GaN film with patterned polarities on a c-Al 2O 3 substrate. We demonstrate that the polarity distribution of GaN based lateral polarity heterostructures can be deduced from the phase image of the piezoresponse with nanometer scale spatial resolution. We also present images of AlN/Si samples with regions of opposite piezoresponse phase, which indicate the presence of antiphase domains. We discuss the potential application of this technique for determination of the orientation of bulk crystals.

Reactive Sputter Deposition of Highly Oriented AlN Films at Room Temperature

Textured as well as epitaxial thin AlN films are of great interest for a wide range of electro-acoustic and optoelectronic applications. Reduction of the deposition temperature is of vital importance in a number of applications due to thermal budget limitations. In this work we systematically studied the influence of the process parameters on the film properties and identified the factors leading to improved film quality as well as reduced deposition temperature with pulsed direct current sputtering in an Ar/N2 atmosphere. We demonstrated that fully textured (0002) films can be grown under a wide range of conditions. At the same time the full width at half-maximum (FWHM) of the rocking curve of the (0002) XRD peak was found to vary systematically with process conditions—depostion rate, process pressure, gas composition, and substrate temperature. The best films showed a FWHM of 1.2°. We found that by far the most important factor is the arrival energy of the sputtered Al atoms, which is primarily controlled by the process pressure. We report for the first time that fully textured AlN films with a FWHM of under 2° can be grown at room temperature. Other important factors are the ion and electron bombardment of the films and substrate temperature as well as gas composition, although their influence is not as dramatic. Generally, the film quality increases with temperature. Bias and electron bombardment within a certain range also lead to better films.

Growth of epitaxial AlN films on (Mn,Zn)Fe 2O 4 substrates by pulsed laser deposition

We have grown AlN on (Mn,Zn)Fe 2O 4 substrates by pulsed laser deposition (PLD) and investigated their structural properties using high resolution X-ray diffraction (HRXRD), reflection high energy electron diffraction (RHEED), and atomic force microscopy (AFM). We have observed the transition of the RHEED pattern from sharp streaks into clear spots at the early stage of the film growth, which indicates that the growth mode of AlN changed from the two-dimensional mode to the three-dimensional mode due to the stress buildup. RHEED and XRD observations have revealed that hexagonal AlN (0 0 0 1) grows on (Mn,Zn)Fe 2O 4 (1 1 1) with the in-plane epitaxial relationship of [1 1 −2 0]AlN//[0 1 −1](Mn,Zn)Fe 2O 4. The lattice mismatch for this alignment is calculated to be 6%. The FWHM value of the AlN (0 0 0 2) X-ray rocking curve is as low as 77 arcsec, which indicates that the density of the threading screw dislocations in the AlN film is quite low.

Periodic oxide breakdown during oxidation of AlN/Sapphire(0001) films

We present an in situ synchrotron x-ray scattering study on the thermal oxidation of epitaxial AlN/Sapphire(0001) films. During annealing to 700 °C, an epitaxial AlN film transforms progressively into a planar epitaxial γ-Al2O3 layer. The oxidation proceeds through the γ-Al2O3/AlN interfacial motion that is observed directly from the intensity fringes near the AlN(0002) Bragg reflection. The oxidation rate, deduced from the interfacial motion, exhibits repeating transitions from a linear to a parabolic oxidation behavior. This suggests that the oxide break down periodically. During the oxidation the strain in the remaining AlN reverts to the value measured at the same thickness during the growth of the AlN film.