AlN Monolayer Research Articles

Alkaline earth metal-doped monolayer AlN: A DFT study of photocatalytic and magnetic properties

The presence of the unintentional impurities C and O has been reported in the preparation of two-dimensional AlN using metal organic vapor phase epitaxy or molecular beam epitaxy under vacuum conditions, whereas the occurrence of VAl is inevitable. Thus, the photocatalytic and magnetic properties of monolayer AlN containing VAl, an unintentional impurity (C/O), and Be/Mg/Ca impurity are studied by using the GGA + U method based on density functional theory. Results indicate that the magnetic moment of Al34OiBeN36 is 2 μB, making it relatively suitable for spintronic devices. The Al34OiCaN36 system has the best visible-light absorption, relatively appropriate band-edge position, and relatively easy carrier separation and migration. Therefore, this system is suitable to be used as a photocatalyst for water decomposition. The results of this study will facilitate the preparation of AlN monolayers and provide insights into their potential applications in the field of spintronics and photocatalysis.

Promising AlN-modified VS2 heterostructure for Li-ion battery anode with high specific capacity

Degradation of the interior structure and the formation of uncontrolled Li dendrites at the negative electrode during the process of detaching and embedding Li atoms in lithium-ion batteries (LIBs) can negatively impact the cycling operation of the battery. To address these shortcomings, we constructed AlN/VS2 heterostructure with the expectation that it can be deployed as anodes in LIBs. Our results show that the AlN/VS2 heterostructure is characterized by strong stability, enhanced electrical conductivity and mechanical properties compared to monolayer AlN and VS2. Small Li clusters and cross-layer adsorption models are not thermodynamically prone to form, implying that the anode material plays a positive role in inhibiting the formation of dendrites. Moreover, the specific capacity of the AlN/VS2 heterostructure is 816.2 mAh/g, which is higher than that of monolayer VS2 (466 mAh/g) and graphene (372 mAh/g). The results of ab-initio molecular dynamics (AIMD) and biaxial stretching further demonstrate the structural and electrochemical stability of the electrode material during the embedding of Li atoms. To sum up, the AlN/VS2 heterostructure show potential as a high-performance anode for utilization in LIBs.

InN/InAlN heterostructures for new generation of fast electronics

N-polar InN/In0.61Al0.39N heterostructures are grown directly on sapphire by using metalorganic chemical vapor deposition. The thickness of Mg-doped In0.61Al0.39N is 340 nm, and the root-mean-square surface roughness of 20 nm thick InN is ∼3.2 nm. An optional AlN spike grown at 710 °C for 35 s is used either as an interlayer to separate the InAlN buffer from the InN channel or as a part of InAlN nucleation after sapphire nitridation. High-resolution transmission electron microscopy reveals approximately two monolayers of AlN if used as the interlayer. In this case, the concentration of screw and edge threading dislocations in partially strained InN decreased down to 6.5 × 109 and 38 × 109 cm−2, respectively. More importantly, the interlayer inclusion suppressed remote donor and alloy disorder scatterings, providing, at room temperature, the InN free electron mobility and concentration of 620 cm2/V s and 3 × 1013 cm−2, respectively. On the other hand, omitting the AlN spike by InAlN nucleation led to structural deteriorations while buffer resistivity increased to 1.7 kΩ/□. A current density of ∼12–16 A/mm, breakdown field of ∼75 kV/cm, and electron drift velocity of ∼2 × 107 cm/s were determined in InN by applying 10 ns voltage pulses on fabricated test resistors.

Strain engineering of electronic, mechanical, and optical properties of orthorhombic III–V group monolayers by first principles calculations

Using first principles calculations, we systematically investigated the effects of strain engineering on the electronic, mechanical, and optical properties of two-dimensional (2D) orthorhombic III–V group materials, including BN, BP, BAs, AlN, AlP, and GaN. It is shown that all the III–V orthorhombic monolayers exhibit excellent mechanical anisotropy for Young’s modulus, Shear modulus, and Poisson’s ratio, especially for the AlN and GaN monolayers. AlN, AlP, and GaN are predicted to be indirect bandgap semiconductors, with their bandgap of 0.70, 0.15, and 0.53 eV, respectively. And BN is demonstrated to be a direct bandgap semiconductor (0.63 eV). Under uniaxial tensile strains, their electronic structures have non-monotonic anisotropic variations and these monolayers can be effectively modulated from metal to semiconductor, experiencing indirect–direct bandgap transitions. In addition, all the orthorhombic III–V materials exhibit highly anisotropic light-harvesting performances and the optical absorbance can be efficiently tailored with tensile strains applied along a- and b-directions. The strong optical absorptions in the visible light regions suggested that AlN, BN, and GaN may be optically tunable 2D materials for component absorbance layers for solar cell applications. The excellent anisotropic and tunable electronic, mechanical, and optical performances indicate that the orthorhombic III–V monolayers are promising candidates for potential applications of optoelectronics and photovoltaics.

Tuning the electronic structure of GaN monolayer by the single-atom promotor for hydrogenation of CO2

The efficient and cost-effective conversion of captured CO2 into valuable chemicals has garnered significant attention, driving the exploration of catalysts for CO2 capture and conversion. Here, we investigated a series of GaN monolayers with single metal atoms (M@GaN, M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, and In) using first-principles calculations and ab initio molecular dynamics (AIMD) simulations. We propose a novel approach to enhance CO2 activation performance by tuning the single atom promotor (SAP). Compared to pristine GaN sheet, Ni@GaN, Cu@GaN, and Zn@GaN are predicted to have higher activity for CO2 adsorption and activation, in which the CO2 chemisorption process is predicted to be exothermic (−0.36 to −0.46 eV) with the low barrier (0.21–0.28 eV). This improvement can be attributed to occupied electronic states near the Fermi level in the N atom connected to SAP in the M@GaN (M = Ni, Cu, and Zn), enabling a better match with the LUMO level of CO2. In addition, compared with the AlN monolayer with better CO2 capture capability, Ni@GaN, and Zn@GaN exhibit considerable promise as catalysts for the hydrogenation of CO2 to HCOOH.

On Decorating a Honeycomb AlN Monolayer with Hydrogen and Fluorine Atoms: Ab Initio and Experimental Aspects.

Mono- and few-layer hexagonal AlN (h-AlN) has emerged as an alternative "beyond graphene" and "beyond h-BN" 2D material, especially in the context of its verification in ultra-high vacuum Scanning Tunneling Microscopy and Molecular-beam Epitaxy (MBE) experiments. However, graphitic-like AlN has only been recently obtained using a scalable and semiconductor-technology-related synthesis techniques, such as metal-organic chemical vapor deposition (MOCVD), which involves a hydrogen-rich environment. Motivated by these recent experimental findings, in the present work, we carried out ab initio calculations to investigate the hydrogenation of h-AlN monolayers in a variety of functionalization configurations. We also investigated the fluorination of h-AlN monolayers in different decoration configurations. We find that a remarkable span of bandgap variation in h-AlN, from metallic properties to nar-row-bandgap semiconductor, and to wide-bandgap semiconductor can be achieved by its hy-drogenation and fluorination. Exciting application prospects may also arise from the findings that H and F decoration of h-AlN can render some such configurations magnetic. We complemented this modelling picture by disclosing a viable experimental strategy for the fluorination of h-AlN.

Ab initio study on electronic structure and magnetism of AlN and InSe monolayer

Based on the first-principle calculations, the magnetism and electronic structures of AlN and InSe monolayers have been investigated. Calculation results show that the system exhibits spin-polarization when the AlN monolayer contains aluminum vacancy (VAl) or nitrogen vacancy (VN), generating magnetic moments of 3.0 μВ and 1.0 μВ, respectively. The formation energy study indicates that VN defects are more likely to be formed than VAl defects. Besides, the 3d transition metal atoms Cr, Mn, Fe, Co, and Ni doped the AlN monolayer, which generated magnetic moments in the order of the atoms 3.0 μВ, 4.0 μВ, 5.0 μВ, 4.0 μВ, and 3.0 μВ, respectively. Mn–Mn is dominated by ferromagnetic coupling at distant distances. Research results express that the InSe film produced a magnetic moment of 1 μВ when the material contained In vacancies (VIn). The magnetism of the film does not change by applying strain.

Anti-corrosive yet color-tunable AlN/Si multilayer hard coatings on 304 stainless steel by magnetron sputtering

AlN based coatings are widely used to protect metal substrates from corrosion. However, synthesizing anti-corrosive yet color-tunable AlN based coatings is challenging. In this study, the AlN/Si multilayer coating was deposited on 304 stainless steel by embedding nine film layers of Si in an AlN monolayer coating using magnetron sputtering. The effects of embedded Si thin layers on the morphology, structure, composition, hardness, friction, optical and corrosion resistance of the AlN coatings were investigated. With the embedding of Si thin layers, the surface pinhole defects and columnar structure of the AlN monolayer coatings were significantly suppressed, and the surface particle size was reduced from 200 to 40 nm, and the average grain size of AlN was reduced from 19.7 to 12.9 nm. The hardness increased from 8.9 to 12.2 GPa, and the friction coefficient decreased from 0.96 to 0.75. The corrosion current density decreased from 6.81 × 10−8 to 2.57 × 10−10 A/cm2 in 3.5 wt% NaCl solution, and from 3.76 × 10−6 to 4.69 × 10−8 in 1 mol/L H2SO4 solution, which is about 4 orders of magnitude lower than the 304 steel. When the thickness of the AlN top layer was controlled to 220, 200, 185, 170, 160, 150 and 140 nm, the colors of the AlN/Si multilayer coatings were red, orange, yellow, green, blue, indigo and violet, respectively. The mechanism of anti-corrosion enhancement and color tunability of AlN/Si multilayer coatings was revealed in detail.

Analysis of AlN monolayer as a prospective cathode for aluminum-ion batteries

Developing cathode materials with high specific capability and excellent electrochemical performance is crucial for the advancement of aluminum-ion batteries, which leverage the high theoretical energy density of aluminum metal anodes. In this paper, we investigated the interaction of cluster and Al atom with AlN (−100) and (001) monolayer using density functional theory to assess the applicability of AlN as cathode material for aluminum-ion batteries. The results show that the AlN (001) monolayer is the most effective for adsorbing and accommodating clusters. Moreover, the AlN (001) monolayer maintains metallic behavior at different concentrations of the cluster, laying the foundation for its battery application. The theoretical storage capacity of the cluster is 105.93 which exceeds that of the Al/graphite battery. The formation energy of -intercalated AlN compounds is −2.74 eV, and the intercalant gallery height is moderate. Furthermore, the diffusion barrier of 0.19 eV for cluster between the AlN (001) monolayer provides high rate capability. The results indicate that AlN monolayer may be a potential cathode material for aluminum-ion batteries.

First-principles study of the effect of alkaline earth metal doping on the magnetic and photocatalytic properties of monolayer AlN: VN-Hi

In the experimental preparation of AlN by chemical vapor deposition, H impurities will inevitably remain, and the intrinsic defect VN will inevitably occur. Incidentally, studies on charged defects are few. In this research, the effect of alkali earth metal doping on the magnetism and photocatalytic properties of single-layer AlN:VN-Hi was studied by first- principles calculations. Al35HiBeN350, Al35HiMgN350, and Al35HiCaN350 have the largest magnetic moments of 1 μB, which are relatively suitable for using as novel two-dimensional spintronic devices. Among all impurity-containing systems, the absorption spectrum of Al35HiCaN350 has the most obvious red shift, with an appropriate band edge position, relatively good carrier activity, difficult recombination of electrons and holes, and the minimum work function. Therefore, single-layer Al35HiCaN350 is the most appropriate photocatalyst for water decomposition given its superior photocatalytic hydrogen evolution. This study is expected to offer theoretical guidance for the development of novel two-dimensional AlN photocatalysts.

Scaling limits of monolayer AlN and GaN MOSFETs

The shrinking of transistors to ultra-scale limits is in great demand to extend Moore’s law, where the search for proper alternative channel materials is significant. The III-V group compounds are regarded as post-Si candidates for their high electron mobility. Herein, we simulate the monolayer (ML) AlN and GaN metal–oxidesemiconductor field-effect transistors (MOSFETs) with ab initio quantum transport simulation to evaluate their scale limit. The ML AlN and GaN MOSFETs exhibit much better n-type performances than their p-type counterparts, and the n-type ML AlN (GaN) MOSFETs can surpass the International Roadmap for Device and Systems (IRDS, 2022 version) lower-power (LP) and high-performance (HP) target for the year 2028 even at gate length (Lg) of 3 and 2 nm, respectively. Encouragingly, the optimal n-type ML AlN (GaN) MOSFETs possess the highest (second-highest) Ion against all studied n-type ML MOSFETs and propel the Lg limit that outperforms the International Technology Roadmap for Semiconductors (ITRS, 2013 version) LP and HP targets to 2 and 1 (2) nm, respectively.

High-order mode solid mounted resonators with polarity inverted multilayered GeAlN/AlN films

High frequency bulk acoustic wave (BAW) resonators for beyond 5 G communications have low Q values and electromechanical coupling because of their ultra-thin piezoelectric monolayer films. Polarity inverted multilayered film BAW resonators operating in high-order mode resonance can have thicker piezoelectric layers than monolayer BAW resonators. In this paper, we fabricated and evaluated two- to four-layered polarity inverted GeAlN/AlN film solid mounted resonators (SMRs). They resonated in high-order mode. Their total film thicknesses were approximately two- to four-times thicker than that of a monolayer AlN film SMR. The polarity inverted GeAlN/AlN film SMRs had higher Q values than the monolayer SMR.

Corrosion resistance of AlN monolayer and Al/AlN multilayer deposited by filtered cathodic vacuum arc

The AlN monolayer and (Al/AlN)5 multilayer coatings were deposited on AZ31 magnesium alloys by filtered cathodic vacuum arc technology. The AlN monolayer showed a dense structure, but there were some holes at the coating/substrate interface. The (Al/AlN)5 multilayer showed a compact layered structure and the holes were interrupted by the Al/AlN interface. Both AlN monolayer and (Al/AlN)5 multilayer provided excellent corrosion resistance for magnesium alloys, with low corrosion current density of 1.91 × 10−6 and 1.11 × 10−6 A/cm2, respectively. Better corrosion resistance of the (Al/AlN)5 multilayer is attributed to its denser layered structure, better adhesion strength, better crack propagation resistance, and deformation coordination between coating and substrate during corrosion.

Ga-doped AlN monolayer nano-sheets as promising materials for environmental sensing applications

Toxic gases such as NO, CO, and SO can cause severe environmental problems. Therefore, research efforts on designing sensors for these gases are imperative. In this study, we have used first-principles calculations to unveil the atomic configurations, adsorption properties, electronic structures, and optical characteristics of these poisonous gases on Ga-doped AlN monolayer nanosheets from a physical perspective. Our findings reveal that the configurations, band structure, adsorption energy, and sensitivity of the nanosheets are significantly affected by the dopant (Ga atom). A physosorption is suggested for CO and NO adsoption system while a chemisorption is characterised for SO system on the Ga-site, indicating their potential as filtration and sensing materials for NO, CO, and SO gases. NO and CO systems are found to show similar optical properties, with absorption peaks in the ultraviolet (UV) region. The refractive index values for NO and CO adsorbants are 1.3 and 3.56 for SO adsorbant on Ga-site, with a slight increase in the visible area. These insights can aid in developing new AlN monolayer-based materials for environmental and sustainability applications.

Optical excitations of graphene-like materials: group III-nitrides.

By using first-principles calculations, we have studied the electronic and optical characteristics of group III-nitrides, such as BN, AlN, GaN, and InN monolayers. The optimized geometry, quasi-particle energy spectra, charge density distributions, band-decomposed charge densities, and Van Hove singularities in density of states are described in the work using physical and chemical pictures and orbital hybridizations found in B-N, Al-N, Ga-N, and In-N chemical bonds. Moreover, the dielectric functions, energy loss functions, absorption coefficients, and reflectance spectra with electron-hole interactions of optical properties are successfully achieved. More importantly, the close relations between electronic and optical properties are successfully demonstrated. The theoretical framework will be useful to research other graphene-like materials.

Buckling effects in AlN monolayers: Shifting and enhancing optical characteristics from the UV to the near visible light range

The structural, electronic, and optical properties of flat and buckled AlN monolayers are investigated using first-principles approaches. The band gap of a flat AlN monolayer is changed from an indirect one to a direct one, when the planar buckling increases, primarily due to diminishing sp2 overlapping and bond symmetry breaking in the conversion to sp3 bonds. The sp3 hybridization thus results in a stronger σ-π bond rather than a σ-σ covalent bond. The calculations of the phonon band structure indicates that the buckled AlN monolayers are structurally and dynamically stable. The optical properties, such as the dielectric function, the refractive index, and the optical conductivity of an AlN monolayer are evaluated for both flat systems and those impacted with planar buckling. The flat AlN monolayer has outstanding optical characteristics in the Deep-UV and absorbs more effectively in the UV spectrum due to its large band gap. The results reveal that optical aspects are enhanced along different directions of light polarization, with a considerable shift in the optical spectrum from Deep-UV into the visible range. Additionally, depending on the polarization direction of the incoming light, increased planar buckling enhances the optical conductivity in both the visible and the Deep-UV domains. The ability to modify the optical and electronic properties of these essential 2D materials using planar buckling technique opens up new technological possibilities, particularly for optoelectronic devices.

Ru doped aluminum nitride monolayer for detecting and scavenging SF6 decomposition components

The SF6 decomposition components detection and elimination are crucial for preventing paralysis of the gas-insulated switchgear (GIS). In this work, Ru doped AlN monolayers with two doping strategies (Ru adatom and Ru substitution) are firstly investigated for selectively sensing and scavenging SF6 decomposition gases (SO2, SOF2, and SO2F2) based on first-principle theory. The most stable structures of Ru adatom and Ru substitution strategies are studied and named Ru-AlN-A and Ru-AlN-S, respectively. The adsorption calculation results show that Ru-AlN-A and Ru-AlN-S possess diacritical adsorption behaviors for SF6 decomposition species. Under the operating temperature, Ru-AlN-A monolayer can serve as the gas sensor or scavenger for selective detecting or removing SO2, SOF2, and SO2F2, whereas Ru-AlN-S monolayer can act as the gas scavenger or one-time gas sensor for SO2 and SOF2 due to the strong adsorption energies. Especially, the adsorption of SO2F2 by Ru-AlN-A makes SO2F2 decompose, while for SOF2, this phenomenon happens when adsorbed by Ru-AlN-S, which is insensitive to SO2F2. The above analysis suggests that Ru-AlN-A and Ru-AlN-S can be explored as novel gas sensors and sweepers for detecting and eliminating the SF6 decomposition species.

Mechanistic insights into low-temperature epitaxial growth of aluminum nitride films on layered transition metal dichalcogenides

It has been demonstrated that the WS2 monolayer is an excellent template for AlN epitaxy at 400 °C low temperature. Low-temperature AlN thin films exhibit much superior crystalline quality than those grown directly on sapphire substrates. In addition to the small lattice mismatch between AlN and WS2 monolayer, we proposed a growth mechanism to explain the excellent van der Waal epitaxy by looking at the initial growth. This growth model reveals that transition metal dichalcogenides (TMDCs) are promising buffer layers for the deposition of III-nitrides but also suggests the novel combination of AlN and TMDCs in the research of future 2D field-effect transistors due to the extremely low leakage current of high-quality AlN films.

High-order mode bulk acoustic wave resonators based on polarity-inverted SiAlN/AlN multilayered films for high-frequency operation

AlN film bulk acoustic wave (BAW) resonators operating at above 5 GHz for next generation mobile communications present some problems, such as the very thin thickness of the piezoelectric film and electrode films. These cause degradations of the power handling capability, electromechanical coupling factor, and Q value in film BAW resonators. Polarity-inverted multilayered AlN film BAW resonators can operate in high-order mode resonance. Therefore, an n-layer polarity-inverted film BAW resonator has n-times thicker piezoelectric film thickness than a standard BAW resonator with a monolayer piezoelectric film operating at same frequency. However, fabrication methods for polarity-inverted multilayered AlN films have not been established. This paper examines the effect of Si doping on AlN films on the crystal orientation, polarity direction, and electromechanical coupling factor (kt2). Furthermore, we fabricated and evaluated two- to eight-layer polarity-inverted SiAlN/AlN film high-overtone bulk acoustic wave resonators (HBARs). The polarity of the SixAl1−xN films inverted around x = 0.024–0.13. The crystal orientation and kt2 of the SixAl1-xN films were degraded with increasing Si concentration x. The eight-layer polarity-inverted SiAlN/AlN film HBAR resonated in the eighth mode. Moreover, the experimental longitudinal wave insertion loss exhibited a similar trend to the theoretical curve calculated by a Mason's equivalent circuit model considering the polarity inverted structure. The eight-layer polarity-inverted HBARs had approximately eight-times thicker piezoelectric film thickness than the monolayer AlN film HBAR. The insertion loss improved with increasing the number of polarity-inverted layers.

Divergence of phonon angular momentum driven by temperature and strain

Phonon angular momentum (PAM) may exhibit exotic temperature dependence as it is sensitive to the phonon lifetime. Constant phonon-lifetime approximation fails to depict such behavior. Here, we study the PAM of AlN, GaN, and graphenelike boron nitride (g-BN) monolayer with full consideration of phonon lifetime using first-principles calculations. We show that wurtzite AlN and GaN acquire divergent PAM at low temperatures from their lowest-lying phonon branches. The g-BN monolayer, on the other hand, does not have finite PAM at equilibrium structure. Rather, it shows intriguing strain dependence on PAM; the compressive strain greater than the critical size generates divergent PAM at low temperatures due to the divergent lifetime of TA phonons. As PAM couples with rotational excitations in solids associated with charge, spin, or electromagnetic fields, our study demonstrates a possibility of mechanical and thermal engineering of such excitations.