AlN Surface Research Articles

Low-temperature growth of stoichiometric aluminum nitride films prepared by magnetic-filtered cathodic arc ion plating

AlN films were prepared on Si(100) and quartz glass substrates with high deposition rate of 30 nm·min−1 at the temperature of below 85 °C by the magnetic-filtered cathodic arc ion plating (FCAIP) method. The as-deposited AlN films show very smooth surface and almost no macrodroplets. The films are in amorphous state, and the formation of AlN is confirmed by N1s and Al2p X-ray photoelectron spectroscopy (XPS). The XPS depth profile analysis shows that oxygen is mainly absorbed on the AlN surface. The AlN film has Al and N concentrations close to the stoichiometric ratio with a small amount of Al2O3. The prepared AlN films are highly transparent over the wavelength range of 210–990 nm. The optical transmission spectrum reveals the bandgap of 6.1 eV. The present technique provides a good approach to prepare large-scale AlN films with controlled structure and good optical properties at low temperature.

In-situ NC-AFM measurements of high quality AlN(0001) layers grown at low growth rate on 4H-SiC(0001) and Si(111) substrates using ammonia molecular beam epitaxy

Nitride wide-band-gap semiconductors are used to make high power electronic devices or efficient light sources. The performance of GaN-based devices is directly linked to the initial AlN buffer layer. During the last twenty years of research on nitride growth, only few information on the AlN surface quality have been obtained, mainly by ex-situ characterization techniques. Thanks to a Non Contact Atomic Force Microscope (NC-AFM) connected under ultra high vacuum (UHV) to a dedicated molecular beam epitaxy (MBE) chamber, the surface of AlN(0001) thin films grown on Si(111) and 4H-SiC(0001) substrates has been characterized. These experiments give access to a quantitative determination of the density of screw and edge dislocations at the surface. The layers were also characterized by ex-situ SEM to observe the largest defects such as relaxation dislocations and hillocks. The influence of the growth parameters (substrate temperature, growth speed, III/V ratio) and of the initial substrate preparation on the dislocation density was also investigated. On Si(111), the large in-plane lattice mismatch with AlN(0001) (19%) induces a high dislocation density ranging from 6 to 12×1010/cm2 depending on the growth conditions. On 4H-SiC(0001) (1% mismatch with AlN(0001)), the dislocation density decreases to less than 1010/cm2, but hillocks appear, depending on the initial SiC(0001) reconstruction. The use of a very low growth rate of 10 nm/h at the beginning of the growth process allows to decrease the dislocation density below 2 × 109/cm2.

Dynamic simulation and experimental study of a novel Al extraction method from AlN under vacuum

AlN chlorination route, in the present work, was proposed to extract Aluminum from aluminum nitride under vacuum. Density functional theory (DFT) were implemented to study the interaction of AlCl3 molecule and AlN (101¯0) surface. The results of DFT indicate that chemisorbed AlCl3 adsorption configuration was observed on the clean AlN (101¯0) surface after structure optimization, and adsorbed AlCl molecules were generated after 1 ps dynamic simulation time. The phase and composition of condensate were examined by means of XRD and EDS. It was found that 97.76 wt% of Al metal was obtained in the experiment B (in the presence of AlCl3), however, no condensate was collected in the experiment A (without AlCl3) at 1760 K under pressure of average 60 Pa. The results show that AlN chlorination route is an alternative Al production method from aluminum nitride.

Improvement of Anti-Hydrolytic Property of AlN Powder Modified by the Rare-Earth Salts

In this article, the high-performance modified AlN powder was prepared, using Y(NO3)3·6H2O as the modifying agent, and characterizing by pH meter, TG , XRD and SEM. The results show that the Y2O3 coating was formed on the AlN surface, the pH value of aluminum nitride of treatment suspension solution maintains 7.75 in water bath for 100 hours. Therefore the conclusion is that surface modification with rare earths can effectively inhibit the hydrolysis of the AlN powder.

Magnetism and electronic structures for B-doped bulk and surface AlN: Ab initio calculations

The magnetism and electronic structures for B-doped bulk and surface AlN are investigated. Our calculations show that the 6.25% B-doped AlN is a half-metallic ferromagnet with a 0.851 eV half-metallic gap. Under an Al-rich condition, the formation energy is 3.83 eV, indicating that the B-doped AlN could be realized experimentally. Electronic structures show that a double-exchange mechanism plays an important role in forming the ferromagnetism. Nitrogen vacancy, emerging in B-doped AlN, would result in a total magnetic moment reduction and half-metallic ferromagnetism loss. For the B-doped surface , calculations show that boron dopants favor the surface layer, and ferromagnetic exchange coupling still exists among the boron dopants.

Reflectance analysis on the MOCVD growth of AlN on Si(111) by the virtual interface model

AbstractThe reflectance‐time profile of AlN‐on‐Si(111) growth by MOCVD as acquired through an in‐situ optical reflectance monitor is analyzed. It is found that, similar to the case of GaN‐on‐sapphire nucleation and growth through the low temperature GaN or AlN buffer layer techniques, the reflectance profile for AlN‐on‐Si also contains information that relate to the growth mechanism, which can then be correlated to the as‐grown material quality. Based on the equations for the Virtual Interface model, a set of 6 parameters can be used to curve‐fit the reflectance traces of AlN growth as acquired by the in‐situ optical monitor. By analyzing a range of 405 nm AlN reflectance‐time data from single layer AlN growth to multilayer AlN/AlGaN/GaN stack grown on Si(111), the “goodness of fit” statistics are found to correlate with material quality metrics such as the XRD (002) rocking curve FWHM for AlN, microscopic morphological roughness of the AlN surface, as well as the resultant wafer bow and curvature for GaN/AlGaN grown on the AlN/Si(111) substrate. The correlations provide useful insight into the desirable mechanism and growth mode for the high temperature MOCVD growth of AlN and GaN on Si(111). When fully established, the technique has the potential of being used as an in‐situ pass/fail screen for the MOCVD growth of GaN‐on‐Si. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Systematic Theoretical Investigations on Surface Reconstruction and Adatom Kinetics on AlN Semipolar Surfaces

The surface reconstructions and adatom kinetics on AlN(1-102) and AlN(1-101) surfaces in the presence of H2 molecules are investigated on the basis of density-functional theory calculations. Our surface phase diagram calculations demonstrate that the H-incorporated surfaces are invariably stable over the wide range of H2 pressure. We find that under the growth condition the adsorption energy of Al adatom on AlN(1-102) surface is much larger than that on AlN(1-101) surface. Furthermore, we find that the diffusion behavior of Al adatom on AlN(1-101) surface is significantly influenced by the presence of hydrogen. These results suggest that impact of hydrogen ambient during the metal-organic vapor-phase epitaxy growth on AlN(1-102) surface is quite different from that on AlN(1-101) surface. [DOI: 10.1380/ejssnt.2015.239]

Reactive Molecular Dynamics Simulations, Data Analytics and Visualization

ABSTRACTMultimillion-atom reactive molecular dynamics (RMD) and large quantum molecular dynamics (QMD) simulations are used to investigate structural and dynamical correlations under highly nonequilibrium conditions and reactive processes in nanostructured materials under extreme conditions. This paper discusses four simulations:1.RMD simulations of heated aluminum nanoparticles have been performed to study the fast oxidation reaction processes of the core (aluminum)-shell (alumina) nanoparticles and small complexes.2.Cavitation bubbles readily occur in fluids subjected to rapid changes in pressure. We have used billion-atom RMD simulations on a 163,840-processor Blue Gene/P supercomputer to investigate chemical and mechanical damages caused by shock-induced collapse of nanobubbles in water near silica surface. Collapse of an empty nanobubble generates high-speed nanojet, resulting in the formation of a pit on the surface. The gas-filled bubbles undergo partial collapse and consequently the damage on the silica surface is mitigated.3.Our QMD simulation reveals rapid hydrogen production from water by an Al superatom. We have found a low activation-barrier mechanism, in which a pair of Lewis acid and base sites on the Aln surface preferentially catalyzes hydrogen production.4.We have introduced an extension of the divide-and-conquer (DC) algorithmic paradigm called divide-conquer-recombine (DCR) to perform large QMD simulations on massively parallel supercomputers, in which interatomic forces are computed quantum mechanically in the framework of density functional theory (DFT). A benchmark test on an IBM Blue Gene/Q computer exhibits an isogranular parallel efficiency of 0.984 on 786,432 cores for a 50.3 million-atom SiC system. As a test of production runs, LDC-DFT-based QMD simulation involving 16,661 atoms was performed on the Blue Gene/Q to study on-demand production of hydrogen gas from water using LiAl alloy particles.

Nitriding of Lightweight High Manganese and Aluminum Steels

Austenitic high manganese and aluminum steels have exceptional combinations of high strength and toughness with excellent wear resistance. Adding aluminum in levels from 6 to 8.8 wt% reduces the density by 10–15 % compared with quenched and tempered Cr and Mo steels but also decreases strain hardening and abrasive wear resistance. Wear resistance may be improved by a low-cost heat treatment in a nitrogen atmosphere to produce a hard layer of surface AlN. In the current study, the effect of aluminum and silicon content on the kinetics of nitriding process was evaluated for a Fe–30 %Mn–(6–9 %) Al–(1–1.6 %)Si–0.9 %C steel in the temperature range of 900–1100 °C (1652–2012 °F). Results show that up to a 550-μm-thick surface layer of AlN plates can be produced, depending on the time and temperature. Increasing the amount of silicon from 1.1 to 1.6 % Si had no statistical effect on the diffusion of nitrogen in the temperature range of 900–1100 °C (1652–2012 °F). Increasing the amount of aluminum from 6 to 8.8 % Al decreased the diffusivity of nitrogen and increased the calculated activation energy from 64 to 79 kJ/mol. The lower than expected values of the activation energy for the diffusion of nitrogen in austenite is suggested to be the result of the development of high-diffusivity pathways at the interface between the AlN and the austenite matrix.

Surface properties of AlN and Al x Ga1−x N epitaxial layers characterized by angle resolved X-ray photoelectron spectroscopy

The AlN and AlxGa1−xN (x = 0.35, 0.73) epitaxial layers were grown on c-plane sapphire substrates by metal–organic chemical vapor deposition. The surface properties of these layers were quantitatively characterized by angle resolved X-ray photoelectron spectroscopy. The characterization results revealed that the air-exposed surfaces of the Al-containing AlN and AlGaN films were covered by a thin Al-oxide layer due to the large chemical affinity of aluminum to oxygen. The as-deposited AlN film was proved to be a nitrogen-deficient film. On the other hand, the Ga Auger effect for the as-deposited AlGaN film was found to be greatly suppressed by increasing Al composition. Moreover, the Al composition in the as-deposited AlGaN films was demonstrated to be non-uniform since Al atom is more easily oxidized than Ga atom, especially for the AlGaN film with relatively high Al content. Our work will be beneficial to understanding and controlling of the surface properties for Al-containing III-nitrides.

The effect of Al interlayers on the growth of AlN on Si substrates by metal organic chemical vapor deposition

The influence of an Al interlayer on the crystalline quality and surface morphology of AlN epitaxial layers is investigated. 100 nm thick (0002) oriented AlN layers were grown on Si (111) substrates by metal organic chemical vapor deposition. Al interlayers were introduced through Al soaking, followed by nitridation for different lengths of time. When the nitridation time was 0 s, the AlN surface was uneven. At a nitridation time of 15 s, the AlN epilayer started to transform into a smooth surface. When the nitridation time was increased to 40 s, the AlN films had rough surfaces. X-ray diffraction measurements showed that the full width at half maximum of the (0002) peak decreased to 801.9 arcsec by introducing the Al interlayer for a nitridation time of 25 s. The roughness of the AlN surface was reduced from 5.30 nm to 1.91 nm compared with the films without an Al interlayer. The Al interlayer improved the crystalline quality and surface roughness of the AlN films. An optimized nitridation time was important in achieving an appropriate surface roughness for high quality crystals of AlN on Si (111) surfaces. Open image in new window

화학적 습식 에칭을 통한 AlN와 GaN의 결함 및 표면 특성 분석

화학적 습식 에칭을 통해 AlN와 GaN의 결함 및 표면 특성을 분석했다. 화학적 습식 에칭은 단결정의 결함을 선택적으로 에칭하기 때문에 결정의 품질을 평가하는 좋은 방법으로 주목 받고 있다. AlN와 GaN의 단결정은 NaOH/KOH 용융액을 이용하여 에칭을 했으며, 에칭 후 표면 특성을 알아보기 위해 주사전자현미경(SEM)과 원자힘 현미경(AFM)을 촬영했다. 에치 핏의 깊이를 측정하여 표면에 따른 에칭 속도를 계산했다. 그 결과 AlN와 GaN 표면에는 두 개의 다른 형태에 에치 핏이 형성 되었다. (0001)면의 metal-face(Al, Ga)는 육각 추를 뒤집어 놓은 형태를 갖는 반면 N-face는 육각형 형태의 소구 모양(hillock structure)을 하고 있었다. 에칭 속도는 N-face가 metal-face(Al, Ga)보다 각 각 약 109배(AlN)와 5배 정도 빨랐다. 에칭이 진행되는 동안 에치 핏은 일정한 크기로 증가하다 서로 이웃한 에치 핏들과 합쳐지는 것으로 보여졌다. 또한 AlN와 GaN의 에칭 공정을 화학적 메커니즘을 통해 알아 보았는데, 수산화 이온(<TEX>$OH^-$</TEX>)과 질소의 dangling bond에 영향을 받아 metal-face(Al, Ga)와 N-face가 선택적으로 에칭되는 것으로 추론되었다. We investigated defects and surface polarity in AlN and GaN by using wet chemical etching. Therefore, the effectiveness and reliability of estimating the single crystals by defect selective etching in NaOH/KOH eutectic alloy have been successfully demonstrated. High-quality AlN and GaN single crystals were etched in molten NaOH/KOH eutectic alloy. The etching characteristics and surface morphologies were carried out by scanning electron microscope (SEM) and atomic force microscope (AFM). The etch rates of AlN and GaN surface were calculated by etching depth as a function of etching time. As a result, two-types of etch pits with different sizes were revealed on AlN and GaN surface, respectively. Etching produced hexagonal pits on the metal-face (Al, Ga) (0001) plane, while hexagonal hillocks formed on the N-face. On etching rate calibration, it was found that N-face had approximately 109 and 15 times higher etch rate than the metal-face of AlN and GaN, respectively. The size of etch pits increased with an increase of the etching time and they tend to merge together with a neighbouring etch pits. Also, the chemical mechanism of each etching process was discussed. It was found that hydroxide ion (<TEX>$OH^-$</TEX>) and the dangling bond of nitrogen play an important role in the selective etching of the metal-face and N-face.

Surface magnetism of Mg doped AlN: a first principle study

Using first-principles calculations, we have systematically investigated the magnetic properties of Mg-doped AlN and surfaces. Both the polar and non-polar surfaces are found to be magnetic and the magnetic moments are mainly due to spin polarized 2p orbitals of surface N atoms surrounding Mg. The splitting of energy levels in both cases favours charge hopping between the minority spin states of N 2p orbitals, which leads to a stable ferromagnetic ground state. However, the range of magnetic coupling and the stability of the ferromagnetic state differ between the polar and non-polar surfaces and are dependent on the nature of localization of the defect states. The ferromagnetic state in a Mg-doped reconstructed surface is more stable than in a Mg-doped AlN surface.

The effect of polarity and surface states on the Fermi level at III-nitride surfaces

Surface states and their influence on the Fermi level at the surface of GaN and AlN are studied using x-ray photoelectron spectroscopy (XPS). The effect of polarity on surface electronic properties was studied. Accurate modeling of the valence band edge and comparison with XPS data revealed the presence of donor surface states at 1.4 eV and acceptor states at energies &amp;gt;2.7 eV from the valence band in GaN. Al polar AlN showed acceptor states at energies &amp;gt;3.3 eV. Density of acceptor surface states was estimated to be between 1013 and 1014 eV−1 cm−2 in both GaN and AlN. The shift in charge neutrality levels and barrier heights due to polarity and the density of surface states on AlN and GaN were estimated from XPS measurements. Theoretical modeling and comparison with XPS data implied full compensation of spontaneous polarization charge by charged surface states. Barrier height measurements also reveal a dependence on polarity with ϕmetal-polar &amp;gt; ϕnon-polar &amp;gt; ϕnitrogen-polar suggesting that the N-polar surface is the most suitable for Ohmic contacts.

Low trap states in in situ SiNx/AlN/GaN metal-insulator-semiconductor structures grown by metal-organic chemical vapor deposition

We report the use of SiNx grown in situ by metal-organic chemical vapor deposition as the gate dielectric for AlN/GaN metal-insulator-semiconductor (MIS) structures. Two kinds of trap states with different time constants were identified and characterized. In particular, the SiNx/AlN interface exhibits remarkably low trap state densities in the range of 1011–1012 cm−2eV−1. Transmission electron microscopy and X-ray photoelectron spectroscopy analyses revealed that the in situ SiNx layer can provide excellent passivation without causing chemical degradation to the AlN surface. These results imply the great potential of in situ SiNx as an effective gate dielectric for AlN/GaN MIS devices.

Significant quality improvement of GaN on Si(111) upon formation of an AlN defective layer

The stress state and crystal quality of a metal–organic chemical vapor deposition (MOCVD)-grown GaN film on a Si(111) substrate with different AlN buffer layer thicknesses were investigated. The properties of the AlN layers with varying thicknesses were studied. The three-dimensional growth mode with a rough surface of AlN on Si was observed. A defective layer with mixed orientations of AlN grains formed at the AlN/GaN interface. The AlN buffer layer with the defective layer accommodated most of the lattice mismatch between the GaN film and the Si substrate. The significance of the defective layer was proved by the obtainment of a 1 μm thick nearly crack-free GaN film with high crystal quality.

Carbon nanotube growth on AlN support: Comparison between Ni and Fe chemical states and morphology

In this work, carbon nanotubes (CNTs) are grown from Ni and Fe nanoparticles supported on a rough AlN surface. Although, identical experimental parameters are used during dewetting (island formation) via thermal treatment, Ni particles appear metallic and larger, whereas Fe particles are smaller and slightly oxidized. This difference in the nanoparticle chemical state and morphology reflects to CNTs during catalytic chemical vapor deposition in terms of their CNT growth mode and size: tip-growth mode for Ni catalyst with CNT diameters of up to 40nm, whereas base-growth mode for Fe with CNT diameters typically less than 10nm are observed.

Growth techniques to reduce V‐defect density in GaN and AlGaN layers grown on 200 mm Si (111) substrate

AbstractV‐defects can be very detrimental to the functionality of GaN based light emitting diodes (LEDs) and high power transistors grown on 200 mm Si (111) substrates. This work focuses on reducing these detrimental defects in the GaN/AlGaN/AlN stacks grown in a Veeco Turbodisc Maxbright MOCVD system. The origins of V‐defects in GaN/AlGaN/AlN material were studied by cross‐sectional transmission electron microscope (TEM) and high angle annular dark field scanning transmission electron microscopy (HAADF‐STEM). It was found that V‐defects were associated with inversion domain boundary (IDB) like defects, which started from the interface between the AlN nucleation layers and the Si (111) substrates. These IDB‐like defects were terminated by generating V‐defects in the upper layers. Encouraging the 2‐D GaN growth mode helped to close these V‐defects in the GaN layers, but relaxed the built‐in compressive stress faster and resulted in excessive wafer bow. Optimizing the V/III ratio during the AlN nucleation step improved AlN crystal quality and surface morphology, and close V‐defects in GaN without degrading wafers stress condition.

Epitaxial growth of homogeneous single-crystalline AlN films on single-crystalline Cu (1 1 1) substrates

The homogeneous and crack free single-crystalline AlN thin films have been epitaxially grown on single-crystalline Cu (111) substrates with an in-plane alignment of AlN [11–20]//Cu [1–10] by pulsed laser deposition (PLD) technology with an integrated laser rastering program. The as-grown AlN films are studied by spectroscopic ellipsometry, field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), polarized light microscopy, high-resolution X-ray diffraction, and high-resolution transmission electron microscopy (HRTEM). The spectroscopic ellipsometry reveals the excellent thickness uniformity of as-grown AlN films on the Cu (111) substrates with a root-mean-square (RMS) thickness inhomogeneity less than 2.6%. AFM and FESEM measurements indicate that very smooth and flat surface AlN films are obtained with a surface RMS roughness of 2.3nm. The X-ray reflectivity image illustrates that there is a maximum of 1.2nm thick interfacial layer existing between the as-grown AlN and Cu (111) substrates and is confirmed by HRTEM measurement, and reciprocal space mapping shows that almost fully relaxed AlN films are achieved only with a compressive strain of 0.48% within ∼321nm thick films. This work demonstrates a possibility to obtain homogeneous and crack free single-crystalline AlN films on metallic substrates by PLD with optimized laser rastering program, and brings up a broad prospect for the application of acoustic filters that require abrupt hetero-interfaces between the AlN films and the metallic electrodes.

Adhesion Enhancement of a Plated Copper Layer on an AlN Substrate Using a Chemical Grafting Process at Room Temperature

AlN substrates have been extensively used for heat dissipation, especially in high-power light-emitting diodes (LEDs). However, the AlN substrate is an electric insulator and thus needs to be metalized to serve as an electric circuit carrier and heat conductor for LED packaging. Herein, we propose a wet process that can be performed at room temperature to metallize AlN substrates. The wet metallization process involves copper electroless deposition and copper electroplating. The catalyst used for the copper electroless deposition was PdCl2, which was chemically grafted onto the AlN surface. The chemical grafting agent was 3-(2-aminoethylamino)propyltrimethoxysilane (APTMS). Before the chemical grafting step, an etching step in KOH solution was critical for good adhesion of the plated copper layer. The electroless copper depositions were catalyzed using chemically grafted Pd and a commercial Sn/Pd colloid. An adhesion comparison test of the plated copper layers was performed to demonstrate that the copper adhesion performance obtained with the chemically grafted Pd process was substantially better than that obtained with the commercial Sn/Pd colloid process.