AlN Thin Films Research Articles

Nitride-Based Microlight-Emitting Diodes Using AlN Thin-Film Electrodes with Nanoscale Indium/Tin Conducting Filaments.

Microlight-emitting diodes (µLEDs) are emerging solutions for both high-quality displays and lighting technologies. However, the overall light output power density of these devices is low, as the emission area is shielded by the p-electrodes required for current injection. In this study, instead of the more conventionally used indium tin oxide (ITO), an AlN thin film with nanoscale conducing filaments (CFs) is used, referred to as CF-AlN, as a transparent conducting electrode (TCE), to enhance the output power density from the same emission area. As a result of this modification, the electroluminescence intensity is enhanced by 10% at an injection current of 10 mA, and the current density is improved by 13% at a forward voltage of 4.9 V, in comparison to the parameters observed with ITO-based µLEDs. This improvement is attributed to the higher transmittance of CF-AlN TCEs, together with efficient hole injection from the p-electrode into the light-emitting layer, through the CFs formed in the AlN layer. In addition, using transmission electron microscopy analyses, the origin of the CFs is directly identified as the diffusion of In and Sn ions, which provides critical insight into the conduction mechanism of AlN-based TCEs.

Thin film bulk acoustic wave filters with ring-dot electrodes

We propose a new structure of ZnO or AlN thin film bulk acoustic wave filters using a ring-dot driving electrode operating with thickness-extensional modes. A theoretical analysis is performed to show the operating frequencies and modes of the proposed filter. The scalar differential equation by Tiersten and Stevens (1983) is used. An analytical solution is obtained. Numerical calculations based on the solution show that the vibration tends to be trapped in the electroded central and annular areas and decays away from the electrode edges. It is also shown that by properly designing the electrode dimensions and mass density, the nodal line of one of the two operating modes can be adjusted to lie in the gap between the ring and the dot driving electrodes which is ideal for a filter.

Thin film bulk acoustic wave piezoelectric resonators with circular ring driving electrodes for mass sensing

We study thickness-extensional vibration frequencies and modes in a thin ZnO or AlN film on a silicon layer with a circular ring driving electrode. It is shown that the vibration tends to be trapped in the annularly electroded area and decays away from the electrode edges. The convexity of the vibration distribution near the film center can be adjusted through design, and can be made nearly uniform for mass sensor application. The theoretical analysis presented can be used to optimize the design of the device.

AlN/GaN metal–insulator–semiconductor high-electron-mobility transistor with thermal atomic layer deposition AlN gate dielectric

In this paper, a novel gate dielectric and passivation technique for GaN-on-Si AlN/GaN metal–insulator–semiconductor high-electron-mobility transistor (MIS-HEMT) is proposed. This technique features the AlN thin film grown by thermal atomic layer deposition (ALD) at 360 °C without plasma enhancement. A 10 nm AlN thin film serving as gate dielectric and passivation layer in the access region was grown. Compared with the Schottky gate AlN/GaN HEMT (SG-HEMT), the fabricated thermal ALD-grown AlN MIS-HEMT exhibits enhanced Ion/Ioff ratio, reduction of gate leakage by 5 orders of magnitude at a bias of 5 V, and suppressed current collapse degradation.

Surface, structural and optical properties of AlN thin films grown on different face sapphire substrates by metalorganic chemical vapor deposition

AlN thin films grown on different face sapphire substrates by metalorganic chemical vapor deposition were studied comprehensively by a variety of techniques including high resolution X-ray diffraction (HR-XRD), X-ray photoelectron spectroscopy (XPS), Raman scattering (RS) and theoretical calculations. Our results showed that substrate orientation has a drastic effect on the initial growth of AlN. The AlN films showed dominant semipolar (101) orientation but with slightly weaker individual features. Optical reflectance measurements confirmed that AlN films grown on different surface orientation sapphire substrates at the same time possessed different film thicknesses. It was found that the surface orientation also has a major influence on the dislocation density, crystallite size, and surface oxygen incorporation. Raman line width exhibited good correlation with XPS measurements. The combined analyses led to a better understanding of the variation in the quality of the AlN films grown on different faces of sapphires.

Effect of growth temperature on AlN thin films fabricated by atomic layer deposition

Resistive random-access memories (RRAM) have been extensively studied because of their advantages such as low operating voltage, high reliability, and simple structure. Among the various types of materials for RRAMs, such as oxides, nitrides, sulfides, and chalcogenides, AlN shows resistive switching phenomena with low energy and high speed via the formation of Al-rich conducting channels owing to the generation of nitrogen vacancies. Moreover, AlN has a large band gap (~ 6.2 eV), high thermal conductivity, and dielectric constant. Therefore, AlN can find application as a gate dielectric material, functional layer, and resistive switching layer for RRAM applications. In this study, AlN thin film is deposited by thermal atomic layer deposition (ALD), which is a self-limiting technique through ligand exchange between the precursor molecules and surface functional groups. We use trimethylaluminum (TMA) and NH3 as the metal precursor and reaction gas, respectively. We obtain growth rates of 0.05–0.16 nm/cycle at a wafer temperature of 274–335 °C. Structural and chemical properties of the AlN films grown at various temperatures are investigated by X-ray diffraction, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. The electrical properties of these AlN films are studied by fabricating the devices having an Al/AlN/Pt stack.

Performance improvements in AlGaN-based ultraviolet light-emitting diodes due to electrical doping effects

We report a new doping method for the fabrication of wide-bandgap (WB) semiconductors such as p-AlGaN using electric fields and the application of this method to AlGaN-based UV light-emitting diodes (LEDs) to evaluate its effect at the device level. We prepared four LED samples with different work function (WF) energies using Pt, Ni, Ti, or Mg as contact metals and applied electric fields between these metals and the p-AlGaN surface across indium-doped tin oxide (ITO)/AlN thin films to facilitate diffusion of the metal atoms into the p-AlGaN layer. Compared to the samples with reference ITO electrodes (10 or 100 nm), ohmic behavior on the p-AlGaN surface was improved in the samples doped with Pt, Ni (high WF), and Mg (low WF but shallow dopant), but not for the sample doped with Ti (low WF). Furthermore, Mg-doped samples exhibited the lowest contact resistance with reasonably high transmittance among the four samples; accordingly, the lowest forward voltage and highest light-output power were achieved with UV LEDs using ITO/AlN/Mg electrodes. This electrical doping method could be useful for WB semiconductors fabricated with materials such as p-AlGaN and p-ZnO, which are difficult to dope using either thermal or optical doping method.

Anisotropic piezoelectric response of ion beam sputtered aluminum nitride thin films textured along [10[formula omitted]0]-axis: A field dependent X-ray diffraction investigation

a-axis oriented AlN thin films sandwiched between Ti-electrodes were prepared on Si (100) substrates using ion beam sputtering in reactive assistance of N2-plasma. Piezoelectric response of such Ti/AlN/Ti heterostructures was investigated using XRD under applied voltage varying from +1 V to +5 V. External electric field induced strain in film microstructure was used to estimate the longitudinal piezoelectric coefficient d33eff. Our results show that AlN thin films with a preferred orientation along a-axis possess enhanced value of d33eff = 560.5±68 pm/V. A phenomenological model is proposed to account this enhancement by considering the dielectric an elastic continuum with Al+— N−— Al+ quadruples as the constituent unit of the Cosserat medium.

Development of AlN thin films for breast cancer acoustic biosensors

Although the development of biosensors has become popular in the recent past, there are still many opportunities to develop specific designs that address public health problems of third world countries. This paper presents the initial efforts toward the development of an affordable acoustic biosensor for breast cancer detection. The core of the sensor consists of a piezoelectric AlN thin film that requires specific crystallographic and morphological features. In this study, the processing–structure relationship of our radio-frequency magnetron sputtering (r.f. MS) system was established. Al/AlN films were produced via r.f. MS varying the applied power and atmosphere composition. The films were analyzed by glancing angle X-ray diffraction, scanning electron microscopy+energy dispersive analysis, X-ray photoelectron spectroscopy, and transmission electron microscopy. The results indicate that applied power had a much stronger influence on the phase selection, orientation and morphology of the films and was attributed to the effect of power on ad-atom mobility on the substrate. Higher power values resulted in films better suited for biosensor applications. The presence of the Al adhesion layer favored the formation of undesirable metastable c-AlN. Preliminary results on the biofunctionalization of the films were encouraging, but require further work both in the protocol and on the effect of the film surface on this process.

Indium hexagonal island as seed-layer to boost a-axis orientation of AlN thin films

Highly a-axis oriented aluminum nitride films have been grown on Indium coated (100) Si substrate by DC reactive magnetron sputtering. It is shown that In incorporated layer improve the extent of preferential growth along (100) axis and form dense AlN films with uniform surface and large grains, devoid of micro-cracks. As revealed by SEM cross section images, AlN structure consists of oriented columnar grains perpendicular to the Si surface, while AlN/In structure results in uniformely tilted column. SEM images also revealed the presence of In hexagonal islands persistent throughout the entire growth. Micro -Raman spectroscopy of the surface and the cross section of the AlN/In grown films evidenced their high degree of homogeneity and cristallinity.

Activating AlN thin film by introducing Co nanoparticles as a new anode material for thin-film lithium batteries

AlN/Co nanocomposite thin films were fabricated by pulsed laser deposition and investigated as new anode materials for lithium-ion batteries for the first time. The combination of electrochemically inactive AlN and Co in nanometer scale boosted the electrochemical performance of the thin films surprisingly. A high reversible capacity of 555 mAh·g−1 after 100 discharge–charge cycles at a current density of 500 mA·g−1 is obtained for the AlN/Co nanocomposite thin films, and 372 mAh·g−1 can be retained at a high rate up to 16C, exhibiting promising cycle stability and rate capability. The electrochemical reaction mechanism study reveals that Co nanoparticles could not only provide high electronic conductivity for the thin films, which facilitate the thorough decomposition of AlN in the initial discharge process, but also react with Li3N to form a new species Co2N during charge process, thus ensuring large capacity and high reversibility of AlN/Co nanocomposite thin films in subsequent cycles. This study provides a new perspective to design advanced electrode materials for lithium-ion batteries.

Study on the Crystallinity Change Caused by Heat Treatment of AlN Thin Films Grown by Using Pulsed Sputter Deposition

Study on the Crystallinity Change Caused by Heat Treatment of AlN Thin Films Grown by Using Pulsed Sputter Deposition

Study of Thin Film Coatings by IR Phonon-Polariton Spectroscopy Methods

The methods of vibrational spectroscopy in far and near fields are used for studying thin (nanometer thick) AlN and MgO films on sapphire. It is shown that in the case of monolayer AlN films formed by the method of sapphire nitridation, the frequencies of longitudinal optical phonons of the film decrease in comparison with those of a single crystal, but the frequencies of transverse optical phonons increase. This means that the crystal structures of the film and the surface of the sapphire substrate modify. The solid solution (Al2O3)1 – x(AlN) x of variable composition appears in the transition layer (at the interface); the parameters of the AlN crystal cell change smoothly. In the case of the epitaxial growth of MgO films on sapphire, during the first stage, most likely, only the crystal structure of the sapphire surface is modified because the frequencies of the optical phonons of MgO films and the single crystal are the same. With an increase in the thickness of the MgO film, tensile stresses are accumulated in it, changing the frequencies of the optical phonons.

Local crystal structure in the vicinity of Cr in doped AlN thin films studied by X-ray absorption spectroscopy.

This article reports the detailed X-ray absorption spectroscopy (XAS) study of Al1-xCrxN (x = 4, 6, 11%) thin films synthesized by the reactive magnetron co-sputtering technique. All these films were crystallized with a hexagonal wurtzite structure with preferential orientation along the a-axis without the formation of any secondary phases. Surface chemical analysis to evaluate the Cr concentration was carried out using X-ray photoelectron spectroscopy. The study confirmed the presence of AlN and Cr in bonding with N. The local crystal structure around the Cr dopant in the as-synthesized and annealed thin films has been analyzed by both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. From XAS, it was found that Cr replaced the Al atom in the AlN lattice and led to a localized CrN species with distorted tetrahedral AlN in the absence of Cr clustering. The bond lengths of (Cr-N)ax, (Cr-N)bs and Cr-Al, extracted from the EXAFS fitting, were found to decrease with the Cr concentration for both the as-synthesized and annealed thin films due to the enhancement of p-d hybridization between the dopant and the host atoms. However, in the annealed 11% Cr film, the bond lengths are larger than the other and tend to match the Cr-N geometry in CrN.

Local crystal structure and mechanical properties of sputtered Ti-doped AlN thin films.

In this article, we predominantly report the investigation of the local crystal structure around a Ti dopant by X-ray absorption spectroscopy (XAS) and the nano-mechanical properties of co-sputtered Al1-xTixN (x = 0 to 4%) thin films. Grazing incidence X-ray diffraction (GIXRD) results show that these films are crystallized with the hexagonal wurtzite structure of AlN. Surface chemical analysis and morphology analysis of Al1-xTixN films are executed using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) techniques, respectively. X-ray absorption near-edge structure (XANES) shows that a Ti atom replaces Al in the AlN crystal and forms localized distorted tetrahedral TiN species, leading to a tensile strain. The bond lengths (Ti-N)ax and (Ti-N)bs are found to be moderately decreased with increasing Ti concentration, which is extracted from the extended X-ray absorption fine structure (EXAFS) analysis. However, the Ti-Al bond length in the second coordination sphere having Al vacancies is unaffected by Ti concentration. The hardness (H) and modulus (E) of Al1-xTixN films are measured by the nano-indentation technique, and increase from 17.5 to 27.6 GPa and 231 to 293 GPa, respectively with x = 0 to 4 at% because of the improvement of p-d hybridization between the Ti and N atoms.

Frequency response improvement of a two-port surface acoustic wave device based on epitaxial AlN thin film

This paper presents an exploration on improving the frequency response of the symmetrical two-port AlN surface acoustic wave (SAW) device, using epitaxial AlN thin film on (0001) sapphire as the piezoelectric substrate. The devices were fabricated by lift-off processes with Ti/Al composite electrodes as interleaved digital transducers (IDT). The impact of DL and the number of the IDT finger pairs on the frequency response was carefully investigated. The overall properties of the device are found to be greatly improved with DL elongation, indicated by the reduced pass band ripple and increased stop band rejection ratio. The rejection increases by 8.3 dB when DL elongates from 15.5λ to 55.5λ and 4.4 dB further accompanying another 50λ elongation. This is because larger DL repels the stray acoustic energy out of the propagation path and provides a cleaner traveling channel for functional SAW, and at the same time restrains electromagnetic feedthrough. It is also found that proper addition of the IDT finger pairs is beneficial for the device response, indicated by the ripple reduction and the insertion loss drop.

Structural, optical, and interface properties of sputtered AlN thin films under different hydrogen dilution conditions

In this work, the influence of different hydrogen dilution conditions on the optical, structural and passivation properties of crystalline, hexagonal aluminum nitride is assessed. The layers were deposited using an inline sputter coater in reactive Ar+N2 and Ar+N2+H2 atmosphere mixtures. Elemental composition was determined using energy dispersive spectroscopy. The structural properties were investigated applying Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy. The optical characterization was performed through transmittance measurements using a modified envelope method. It could be observed that the incorporation of hydrogen leads to an increase of crystalline texture, grain size and bandgap. The full-width at half-maximum of the A1 transverse optical phonon mode decreases with increasing grain size and optical bandgap induced by the deposition conditions, showing a good correlation between the optical and crystalline properties. The potential of aluminum nitride for surface passivation of silicon is discussed in terms of surface recombination velocity, fixed charge density and defect state density at the c-Si/AlN:H interface.

Preparation and characterization of Er-doped AlN films by RF magnetron sputtering

Er-doped AlN thin films were deposited by RF magnetron sputtering on (0001) sapphire substrates under different temperature. We systematically investigate the influence of substrate temperature on the crystalline structure and the piezoelectric properties of the films. Consequently, the XRD intensity of (0 0 2) oriented peak first increases and then decreases with increasing substrate temperature, reaching a maximum value and a highly c-axis columnar crystal structure at 200 °C. The piezoelectric constant d33 indicates a maximum value of 9.41 pm/V at substrate temperature of 200 °C.Due to Er doping in AlN films, an improvement in their crystalline structures and piezoelectric properties is noticed.

Influence of N2/H2 and N2 plasma on binary III-nitride films prepared by hollow-cathode plasma-assisted atomic layer deposition

The authors reported the hollow-cathode plasma-assisted atomic layer deposition of AlN, GaN, and InN films using N2-only and N2/H2 plasma. In this study, the authors analyzed the effect of plasma gas composition on the properties of deposited binary III-nitride thin films. Toward this goal, AlN, GaN, and InN films were deposited on Si (100) substrates using N2-only (50 sccm), as well as N2/H2 (50 + 50, 50 + 25 sccm) plasma to investigate the impact of H2 flow. Grazing-incidence x-ray diffraction (GIXRD) patterns of AlN and GaN thin films deposited with N2/H2 plasma remained almost unchanged when H2 flow decreased from 50 to 25 sccm. On the other hand, the use of N2 plasma without any H2 resulted in amorphous GaN thin films with significant carbon impurity within the bulk film. In the case of AlN, similar behavior was observed as the crystal structure is significantly altered to amorphouslike material. Thicknesses of AlN and GaN thin films increased tremendously when N2-only was used as the plasma gas. Furthermore, refractive index values of both AlN and GaN films decreased upon the use of N2-only plasma, which confirm the deterioration of the film quality. Structural weaknesses of GaN and AlN films deposited with N2-only plasma are due to presences of carbon impurities that are trapped inside the growing film. Interestingly, the authors did not observe similar results in InN films grown with N2/H2 plasma. For InN, GIXRD and spectroscopic ellipsometry results show that the phases of deposited films change from InN to In+InN as H2 content in the plasma gas is increased. On the other hand, InN films grown with N2-only plasma show improved structural properties. However, significantly higher N2 plasma exposure times are needed to minimize the residual carbon content in deposited InN layers.

Electric-dipole absorption resonating with longitudinal optical phonon-plasmon system and its effect on dispersion relations of interface phonon polariton modes in metal/semiconductor-stripe structures

Interface phonon polaritons (IPhPs) in nano-structures excluding metal components are thoroughly investigated because they have lower loss in optical emission or absorption and higher quality factors than surface plasmon polaritons. In previous reports, it is found that strong infrared (IR) absorption is based on the interaction of p-polarized light and materials, and the resonance photon energy highly depends on the structure size and angle of incidence. We report the optical absorption by metal/semiconductor (bulk-GaAs and thin film-AlN)-stripe structures in THz to mid-IR region for the electric field of light perpendicular to the stripes, where both of s- and p-polarized light are absorbed. The absorption resonates with longitudinal optical (LO) phonon or LO phonon-plasmon coupling (LOPC) modes, and thus is independent of the angle of incidence or structure size. This absorption is attributed to the electric dipoles by the optically induced polarization charges at the metal/semiconductor, heterointerfaces, or interfaces of high electron density layers and depression ones. The electric permittivity is modified by the formation of these dipoles. It is found to be indispensable to utilize our form of altered permittivity to explain the experimental dispersion relations of metal/semiconductor-IPhP and SPhP in these samples. This analysis reveals that the IPhPs in the stripe structures of metal/AlN-film on a SiC substrate are highly confined in the AlN film, while the permittivity of the structures of metal/bulk-GaAs is partially affected by the electric-dipoles. The quality factors of the electric-dipole absorption are found to be 42–54 for undoped samples, and the value of 62 is obtained for Al/AlN-IPhP. It is thought that metal-contained structures are not obstacles to mode energy selectivity in phonon energy region of semiconductors.