Crystalline AlN Films Research Articles

Polarity tuning of crystalline AlN films utilizing trace oxygen involved sputtering and post-high-temperature annealing

The polarity tuning of AlN films via introducing a period of trace oxygen supply in the initial sputtering process was demonstrated. High-temperature annealing was further implemented to improve the crystal quality of AlN. High-quality pure N-polar AlN film could be achieved by sputtering with pure N2 gas. When trace oxygen was intentionally inputted in the sputtering chamber and its supply time surpassed 150 s, the AlN surface polarity transmitted to pure Al-polar. After optimization, the full widths at half of maximum of X-ray rocking curves for (0002)/(10-12) reflection were improved to 41.3/132.5 arcsec for N-polarity and 38.2/158.7 arcsec for Al-polarity.

Preparation of single crystalline AlN thin films on ZnO nanostructures by atomic layer deposition at low temperature

The growth of hetero-epitaxial ZnO-AlN core–shell nanowires (NWs) and single crystalline AlN films on non-polar ZnO substrate at temperature of 380 °C by atomic layer deposition (ALD) was investigated. Structural characterization shows that the AlN shells have excellent single-crystal properties. The epitaxial relationship of [0002]ZnO//[0002]AlN, and [10−10]ZnO//[10−10]AlN between ZnO core and AlN shell has been obtained. The ZnO NW templates were subsequently removed by annealing treatment in forming gas, resulting in ordered arrays of AlN single-crystal nanotubes. The impact factors on the epitaxial growth of AlN films are thoroughly investigated. It turned out that the growth parameters including lattice mismatch between substrate and AlN, growth temperature, and the polarity of ZnO substrate play important roles on the growth of single-crystal AlN films by ALD. Finally, non-polar AlN films with single-crystalline structure have been successfully grown on m-plane ZnO (10−10) single-crystal substrates. The as-grown hollow AlN nanotubes arrays and non-polar AlN films with single-crystalline structures are suggested to be highly promising for applications in nanoscale devices. Our research has developed a potential method to obtain other inorganic nanostructures and films with single-crystalline structure at fairly low temperature.

Low temperature deposition of high quality single crystalline AlN thin films on sapphire using highly oriented monolayer MoS2 as a buffer layer

AlN films are commonly deposited on sapphire substrates. However, the growth process usually requires high temperature and multi-steps to improve the film quality, due to the large lattice mismatch between the film and substrate. Here we demonstrate that using monolayer MoS2 grown on sapphire as a template, single crystalline AlN films with high crystallinity can be obtained at a much lower growth temperature using helicon sputtering system. The x-ray rocking curve of AlN films prepared on MoS2/sapphire template at 400 °C shows a full width at half maximum of 0.050°, showing a dramatic reduction compared to that of 0.803° for those grown directly on sapphire substrate. The estimated dislocation density of AlN on MoS2/sapphire is decreased to 7.7x107 cm−2, which is comparable to that prepared by metal-organic chemical vapor deposition at high temperatures. The small lattice mismatch between MoS2 and AlN, as well as the polar crystal of monolayer MoS2 apparently improve the quality of AlN films and facilitate AlN formed at lower temperature.

Electrical characterization of high k-dielectrics for 4H-SiC MIS devices

We report promising results regarding the possible use of AlN or Al2O3 as a gate dielectric in 4H-SiC MISFETs. The crystalline AlN films are grown by hot wall metal organic chemical vapor deposition (MOCVD) at 1100 °C. The amorphous Al2O3 films are grown by repeated deposition and subsequent low temperature (200 °C) oxidation of thin Al layers using a hot plate. Our investigation shows a very low density of interface traps at the AlN/4H-SiC and the Al2O3/4H-SiC interface estimated from capacitance-voltage (CV) analysis of MIS capacitors. Current-voltage (IV) analysis shows that the breakdown electric field across the AlN or Al2O3 is ∼ 3 MV/cm or ∼ 5 MV/cm respectively. By depositing an additional SiO2 layer by plasma enhanced chemical vapor deposition at 300 °C on top of the AlN or Al2O3 layers, it is possible to increase the breakdown voltage of the MIS capacitors significantly without having pronounced impact on the quality of the AlN/SiC or Al2O3/SiC interfaces.

Exfoliation of AlN film using two-dimensional multilayer hexagonal BN for deep-ultraviolet light-emitting diodes

Crack-free crystalline AlN film was synthesized on two-dimensional multilayer hexagonal BN (h-BN) by metal organic vapor phase epitaxy (MOVPE). The multilayer h-BN was directly grown on sapphire substrates in wafer scale by MOVPE, with a thickness of 2.9 nm. The AlN film grown on the h-BN/sapphire presented a smooth surface. We further realized the exfoliation of AlN film utilizing the weak bonds within h-BN layers. Moreover, the AlGaN-based deep-ultraviolet light-emitting diodes grown on the AlN/h-BN/sapphire template exhibited obvious emissions at 281 nm. It is promising that the multilayer h-BN will pave the way to obtain transferable high-efficiency devices with wafer scale.

Reactive chemical vapor deposition of heteroepitaxial Ti1−xAlxN films

A novel methodology combining CVD experiments, nanoscale characterisation and reaction–diffusion modelling demonstrates Ti1−xAlxN epitaxial growth on single crystalline AlN films.

Hard AlN films prepared by low duty cycle magnetron sputtering and by other deposition techniques

Crystalline AlN films are very attractive due to their properties such as high thermal stability and relatively high hardness and piezoelectric response. However, the deposition of dense textured AlN films with superior quality at a high deposition rate remains a challenge. In the present work, a reactive low duty cycle pulsed direct current magnetron sputtering (LDMS) process was employed to deposit AlN films on glass and silicon substrates. An arc-free discharge on the Al target was achieved by using short voltage pulses of 10 μs at a low duty cycle of 10%. The authors optimized the deposition conditions in terms of reactive gas flow, working pressure, average target power, substrate temperature, substrate bias, and the level of target erosion. With the optimized deposition conditions, the authors were able to obtain transparent crystalline AlN films with strong (002) preferential orientation and very good optical and mechanical properties: The AlN films with the highest refractive index of 2.1 present ...

Growing aluminum nitride films by Plasma-Enhanced Atomic Layer Deposition at low temperatures

Aluminum nitride films have been grown by Plasma-Enhanced Atomic Layer Deposition method. It was found that at temperatures of 250 °C and 280 °C increase of the plasma exposure step duration over 6 s, as well as increase of reactor purge step duration over 1 s does not affect the growth rate, however, it affects the microstructure of the films. It was found that crystalline aluminum nitride films deposit with plasma exposure duration over 10 s and the reactor purging over 10 s. When the temperature drops the increase of reactor purge step duration and plasma exposure step duration over 20 s is required for crystalline AlN film growth.

Liquid Phase Epitaxy of Si-Doped AIN at 1300 °C in Ga-Al Melt

In the present study, we have successfully grown Si-doped AlN developed by solution growth technique using Ga-Al melt as a solvent under nitrogen atmosphere at 1300 °C. Si doping was introduced to the Ga-Al melt by adding pure Si metal. To allow homoepitaxial growth during solution growth experiment, sapphire substrate were nitrided with precise control to produce hiqh quality single crystalline AlN films with low dislocation density. With the help of AlN film template from above methods, we have successfully grown Si-doped AlN single crystalline layer with a flat surface and almost free from cracks. The full width at half maximum (FWHM) of x-ray rocking curve values for (0002) and (10-12) diffraction from the Si-doped AlN film were 43,2 and 594 arcsec, respectively.

Epitaxial growth of GaN films on Si(110) substrates by rf‐MBE

AbstractGaN films grown by rf‐MBE on Si(110) substrates using AlN buffer layers are investigated. Single crystalline AlN films are obtained by Al pre‐deposition during the buffer layer growth. 2‐dimensional epitaxial growth mode of GaN is confirmed by in‐situ RHEED observations. HRXRD results show that the FWHM values of diffraction peaks decrease with the increase of the film thickness. Atomically flat GaN surface morphology with mono‐layer steps is obtained and crack‐free GaN films until 2 μm are successfully achieved. Micro‐Raman measurements show that the tensile strain in the GaN films on Si(110) substrates is reduced compared to those on conventional Si(111) substrates. It is expectable that high‐quality III‐nitrides can be grown by rf‐MBE technique for device applications. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Role of charge particles irradiation on the deposition of AlN films using plasma focus device

Highly crystalline AlN films are deposited on Si substrate by the bombardment of charge particles emanating from plasma focus device. Deposition of AlN films are carried out for multiple focus shots at room temperature. Numbers of ions (1.3×10 19–5.6×10 19 m −3) are emitted during one focus shot. X-rays diffraction results reveal that an amorphous and crystalline AlN films are obtained for 1 and 3 focus shots, respectively. Crystallinity of AlN film increases up to 5 focus shots while it starts to decrease for 15 and 20 focus shots. The average crystallite size of AlN (1 0 0) phase increases from 33 to 140 nm for 3–5 focus shots, whereas it starts to decrease from 26 to 22 nm for 15–20 focus shots. Raman's spectroscopy exhibits the presence of A 1 (TO) and E 1 (TO) modes at ∼ 611 and ∼ 665 cm −1 positions respectively confirming the formation of AlN film. Fourier's transform infrared spectroscopy indicates the presence of strong absorption peak at ∼ 667 cm −1 position. Field emission scanning electron microscopic investigation reveals that different surface morphologies for different focus shots are observed which are mesh like but uniform for 1 shot, dense microstructure (rounded grains) for 3 shots, rough surface (large particulates) for 5 shots, rounded grains but dense microstructure for 15 shots and multi-layers formation showing rounded grains for 20 shots. Thickness of AlN film deposited for 1 focus shot is uniform and is about 190 nm. Energy dispersive X-ray spectroscopic results show the presence of Al and N elements in AlN film deposited for 5 focus shots.

Influences of repetition rate of laser pulses on growth of crystalline AlN films on sapphire(0001) substrates by pulsed laser deposition

We investigated influences of a repetition frequency of laser pulses on growth of AlN crystalline films by pulsed laser deposition. The structural and morphological properties of the films were studied by X-ray diffraction and scanning electron microscopy. Employment of high frequency laser pulses not only enhanced the growth of AlN crystallites, but also afforded the crystal growth at higher nitrogen pressures. Growth of α-AlN was dramatically enhanced with an increase in the laser pulse frequency, while β-AlN was grown at the high frequency of laser pulses and high nitrogen pressures.

Elastic buckling of AlN ribbons on elastomeric substrate

For optoelectronic applications requiring unconventional substrates, use of flexible forms of semiconductors may be inevitable. We have fabricated a flexible form of single crystalline AlN ribbons with periodic and wavelike structures on an elastomeric substrate. Single crystalline AlN films were grown on Si (111) substrate using high vacuum unbalanced magnetron sputtering. Crystallinity of the AlN films was confirmed with x-ray diffraction and pole figure. The AlN ribbons were transferred to a prestrained (3.1%) elastomeric substrate and the resulting “wavy” ribbons on the substrate were flexible up to 30% strain without any crack formation on the surface. As the film thickness changed from 300 to 900 nm, the periods of the waves varied from 95 to 277 μm, which were within a 10% error of the calculated values. When we applied more than 30% strain, mechanical instability, such as the failure of AlN ribbons due to the cracks on the surface and the merged ribbons, was observed.

Characteristics of single crystalline AlN films grown on Ru(0 0 0 1) substrates

We have grown AlN films on Ru(0 0 0 1) substrates using a low-temperature growth technique with pulsed laser deposition. We found that AlN(0 0 0 1) grows epitaxially on Ru(0 0 0 1) with an in-plane epitaxial relationship of AlN[ 1 1 2 ¯ 0 ]//Ru[ 1 1 2 ¯ 0 ]. Electron backscattering diffraction observations revealed that neither 30° rotational domains nor cubic phase domains were present in the AlN films and the full-width at half-maximum of the distribution in the AlN[0 0 0 1] crystalline orientation was 0.56°. Spectroscopic ellipsometry measurements showed that the AlN/Ru hetero-interface was quite abrupt, which is important for fabrication of high-frequency film bulk acoustic resonators.

Synthesis of Ti–Al–Si–N nanocomposite films using liquid injection PECVD from alkoxide precursors

A series of nitride films of the Ti–Al–Si system have been synthesized at about 750 °C using the liquid injection plasma-enhanced chemical vapor deposition technique from corresponding alkoxide precursors. The composition and structure of the films were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, and scanning and transmission electron microscopy. Amorphous SiN x and crystalline AlN and TiN films were formed in their individual cases, while the SiN x -containing (Ti/Al)Si–N composite films showed a two-phase microstructure consisting of amorphous SiN x matrix and embedded TiN, AlN, or TiAlN nanocrystals. The Si-rich nanocomposite films demonstrated superior high-temperature oxidation resistance. After two hours of air exposure at 900 °C, no elemental segregation was observed in the nitride films and their structures remained basically unaffected. The fundamental mechanism is discussed in terms of the chemically high inertness of amorphous SiN x interfacial phase and thereby the high thermal stability of the composite nitride films with unique nanostructures.

Preparation and Magnetotransport Properties of Alpha-Fe Nanoparticles Dispersed in AlN Granular Films

The structure and magnetotransport properties of alpha-Fe (hereafter denote as α-Fe) nanoparticles in reactively cosputter-deposited Al1-xFexN films (0.18≤x≤0.91) were studied after their annealing. α-Fe-dispersed crystalline AlN films were obtained by annealing in the temperature range between 300°C and 500°C in hydrogen. A magnetoresistance of 0.82% at room temperature was observed on the granular film annealed at 400°C. This value increased to 2.16% at 77 K.

Low temperature deposition of crystalline AlN films by bias-enhanced Cat-CVD

In this work, crystalline AlN films were successfully synthesized on Si (100) by bias-enhanced catalytic chemical vapor deposition (Cat-CVD) at a low substrate temperature of 400 °C. The films were characterized by Fourier Transformation Infrared (FTIR) spectroscopy and X-ray diffraction (XRD). The results indicate that the bias plays an important role in enhancing the crystallinity of the AlN films. Furthermore, it was observed that the crystalline AlN films with small grains of 20 nm could be obtained under the negative bias of 400 V. The effects of the bias on the growth of the crystalline films and the grain size are discussed in detail.

Ligand stabilised dialkyl aluminium amides as new precursors for aluminium nitride thin films

Ligand stabilised aluminium amides of the type Me2Al[NR′(CH2)2NR2] where R = Me and R′ = Et (1) are synthesised using HNEt(CH2)2NMe2 (1a) via an alkane elimination reaction, with an equimolar mixture of Lewis acid (trimethylaluminium) and Lewis base (substituted ethylenediamine 1a), in the absence of solvent. The product is a colourless liquid having relatively high vapour pressure as indicated by TGA. Metalorganic chemical vapour deposition (MOCVD) of the precursor in the presence of ammonia carried out in temperature range of 500–950 °C resulted in crystalline AlN films. The self decomposition (without ammonia) of the precursor under MOCVD conditions resulted in aluminium (carbo) nitride films having the composition Al∶N∶C 1∶0.7.0.6 (±0.1) above 600 °C using N2 and H2 as carrier gas.

Single crystalline AlN film formed by direct nitridation of sapphire using aluminum oxynitride buffer

A noble method forming single crystalline AlN films has been developed as a new substrate for blue/UV light emitters. Sapphire substrates have been nitrided by appropriate CO–N 2 gas mixtures saturated with graphite based on the chemical potential diagram of the Al–N–O–C system. The nitrided surface of sapphire consists of consecutive layers of AlN and γ-aluminum oxynitride (γ-ALON) with low-level dislocation density, where the γ-ALON layer spontaneously forms as an equilibrium phase and acts as a buffer. The lattice mismatch between sapphire substrate and AlN layer has been effectively reduced by using the γ-ALON buffer, which significantly attributes to the growth of single crystalline AlN.

Structural properties of aluminum–nitrogen films prepared at low temperature

Aluminum–nitrogen thin films have been obtained under well-controlled conditions by ion-beam-assisted deposition (IBAD). The films were deposited on crystalline silicon and sapphire substrates at relative low temperature (∼325 K). Taking advantage of the ion energy control provided by the IBAD technique, the films were prepared with N2+ ions with energies ranging from 100 to 300 eV. After deposition, the films were investigated by in situ x-ray photoelectron spectroscopy, ex situ by optical spectroscopy, and x-ray diffraction. Detailed Raman scattering measurements in the 100–2500 cm−1 wave number range were also performed revealing interesting features related to the atomic composition and structure of the films. The Raman data suggest that a misidentification of some vibration modes can lead to incorrect interpretations of the crystalline quality of aluminum–nitrogen films. Finally, the results indicate the suitability of IBAD to produce crystalline AlN films at considerably lower temperatures.