AlN Layer Research Articles

A 326 nm pure ultraviolet light sources achieved in a Ga[formula omitted]O[formula omitted] microwire heterojunction through interface optimization and surface-modification

Low-dimensional Ga2O3 single-crystals have caused widespread attentions in the potential of developing ultraviolet light sources due to its ultrabroad bandgap, low-budget, high thermally stable, high breakdown voltage and others. Herein, we exhibit a significant route to construct one-dimensional Ga2O3 microwire heterojunction deep-ultraviolet light-emitting diode (LED). The resulting electroluminescence is positioned at 326 nm with a narrow linewidth of about 22.5 nm, which is the shortest wavelengths once published for the Ga2O3-related light sources. In the well-fabricated LED, an intermediate AlN layer enables appropriately to manipulate band alignment of n-Ga2O3/p-GaN heterojunction, thus achieving efficient confinement of electron–holes spreading and radiative recombination within the Ga2O3 microwire active medium. Benefited from surface-modified Pt nanoparticles, the LED’s electroluminescence efficiency and brightness can be plasmonically boosted. More importantly, the peak energy at 3.8 eV obtained under forward bias, which is smaller than the Ga2O3 bandgap, could be resulted from radiative recombination of weakly-bounded electron–holes inside the Ga2O3 microwires. Therefore, the Ga2O3 monocrystalline wires synthesized using high-temperature carbothermal reduction, are expected as promising-looking candidates to develop environmentally friendly, easy miniaturization, highly-stable deep-ultraviolet light sources and photonic devices.

Relation of V/III ratio of AlN interlayer with the polarity of nitride

Abstract N-polar GaN film was obtained by using a high-temperature AlN buffer layer. It was found that the polarity could be inverted by a thin low-temperature AlN interlayer with the same V/III ratio as that of the high-temperature AlN layer. Continuing to increase the V/III ratio of the low-temperature AlN interlayer, the Ga-polarity of GaN film was inverted to N-polarity again but the crystal quality and surface roughness of GaN film greatly deteriorated. Finally, we analyzed the chemical environment of the AlN layer by x-ray photoelectron spectroscopy (XPS), which provides a new direction for the control of GaN polarity.

Low-temperature formation of Ti2AlN during post-deposition annealing of reactive multilayer systems

Mn+1AXn-phase Ti2AlN thin-films were synthesized using reactive sputtering-based methods involving the deposition of single-layer TiAlN, and Ti/AlN and TiN/TiAl multilayers of various modulation periods at ambient temperature and subsequent annealing at elevated temperatures. Ex situ and in situ x-ray diffraction measurements were used to characterize the Ti2AlN formation temperature and phase fraction. During annealing, Ti/AlN multilayers yielded Ti2AlN at a significantly lower in situ temperature of 650 °C compared to TiN/TiAl multilayers or single-layer TiAlN (750 °C). The results suggest a reactive multilayer mechanism whereby distinct Ti and AlN layers react readily to release exothermic energy resulting in lower phase transition temperatures compared to TiN and TiAl layers or mixed TiAlN. With a modulation period of 5 nm, however, Ti/AlN multilayers yielded Ti2AlN at a higher temperature of 750 °C, indicating a disruption of the reactive multilayer mechanism due to a higher fraction of low-enthalpy interfacial TiAlN within the film.

The role of grain boundary and alloy scattering within the Callaway model to calculate lattice thermal conductivity in GaN/AlN superlattice

The grain boundary and alloy scattering within the Debye-Callaway model were introduced to calculate the temperature-dependent lattice thermal conductivity (LTC) in the GaN/AlN superlattice. The superlattice layers are assumed to be grains, and the sample is then treated as a multigrain material. The computations include various physical parameters related to Aluminum alloy compositions, such as Debye temperature, atomic mass, lattice volume, density, alloy scattering factor and deformation energy. In general, the sample size effect on LTC in this superlattice structure was similar to any single component solids, but it has a significant influence from the grain boundaries represented by the thickness of layers in the form of LTCPeakpoint.=5.9925e0.0005L2 at the peak point maximum and LTC(600K)=0.344L at 600K, L is the sample size. The dislocations in these samples are controlled by the inbuilt AlN layers with the dependence of NDisl.=−0.542LAlN+7.4.

Metalorganic Vapor‐Phase Epitaxy of +c/−c GaN Polarity Inverted Bilayer for Transverse Quasi‐Phase‐Matched Wavelength Conversion Device

Photon‐pair generation based on optical parametric down‐conversion has attracted for the application as a light source for quantum information. Highly efficient wavelength‐conversion devices require a polarity‐inversion structure when using nitride semiconductors. A transverse quasi‐phase‐matching (QPM) polarity‐inverted GaN bilayer channel waveguide device is suitable for efficient wavelength conversion. This study designed a cross‐section device to satisfy the modal dispersion phase‐matching condition between the TM02 mode pump light and the TM00 mode signal/idler light. Moreover, an AlN oxidation interlayer fabricates the Ga‐polar/N‐polar (+c/−c) GaN layers via metalorganic vapor‐phase epitaxy (MOVPE). A 145 nm thick film layer with a macro‐step‐free surface is grown by optimizing the −c‐GaN growth conditions and reducing the substrate off‐angle to 0.2°. Next, the AlN layer is oxidized in an electric furnace and MOVPE is used to regrow a 1500 nm thick +c‐GaN layer. A macrosteps‐free surface can be achieved by reducing the off‐angle to 0.2° and optimizing the −c‐GaN growth conditions to avoid hillock formation. These results pave the way for improving the efficiency of GaN transverse QPM wavelength‐conversion devices.

Impact of thick N-polar AlN growth on crystalline quality and electrical properties of N-polar GaN/AlGaN/AlN FET

In this study, we attempted to fabricate N-polar GaN/AlGaN/AlN heterostructure FET by changing the thickness of the AlN layer. An Al-polar tiny-pit AlN layer and a polarity inversion method were used to grow N-polar AlN on vicinal sapphire via the metal-organic vapor phase epitaxy. The samples with an AlN thickness of up to 3.4 μm exhibited a crack-free surface. Additionally, the twist component of the crystal quality improved with an increasing AlN thickness. Consequently, the mobility, sheet conductivity, and surface flatness improved. The FET fabricated from the sample with a thicker AlN layer achieved a higher drain current of 279 mA mm−1 at a gate bias of V G = 3 V.

Tackling residual tensile stress in AlN-on-Si nucleation layers via the controlled Si(111) surface nitridation

This work is devoted to the study of the influence of controlled Si(111) surface nitridation on the epitaxial growth of AlN-on-Si nucleation layers with reduced tensile stress on ordered crystalline silicon nitride phase. The Si(111) surface nitridation process was performed at low ammonia flux and substrate temperatures in the range of 700–900 °C and was studied using RHEED and STM techniques. A universal criterion, namely the stage of the nitridation process completion is introduced, taking into account the influence of substrate temperature, ammonia flux and nitridation time. The 100 nm AlN nucleation layer on silicon substrates grown by ammonia molecular beam epitaxy is studied using AFM, XRD, HR-TEM and Raman spectroscopy techniques. The Raman data show that reducing the nitridation temperature from 900 °C to 700 °C not only deteriorates the crystalline quality of the subsequent AlN nucleation layers, but also reduces the residual tensile stress by almost 30 %. In the present contribution, micro-Raman spectroscopy is used to determine the nature of the defects formed during the high temperature growth of the AlN nucleation layers and confirms them to be inversion domains. The HR-TEM technique was used to study the AlN/Si interface in AlN-on-Si nucleation layers grown on a nitridated silicon surface at 700 °C and 900 °C at the optimum stage of the nitridation process completion. HR-TEM images of AlN nucleation layers revealed regions with different AlN/Si(111) interfaces: 1) AlN/amorph-Si3N4/Si, 2) AlN/SiN(8 × 8)/Si, and 3) AlN/Si with a sharp interface boundary. Using fast Fourier transform image analysis, it is shown that the presence of amorphous Si3N4 phase inclusions in the AlN/Si interface boundary introduces tensile stresses in the AlN nucleation layer which can be reduced by lowering the nitridation temperature. The results obtained clearly show that one of the causes of cracks in III-nitride layers grown on silicon substrates is the formation of tensile AlN layers with a high content of the amorphous Si3N4 phase at the AlN/Si interface, which is characteristic of silicon nitridation at elevated temperatures (> 700 °C).

First-Principles Investigation on the Tunable Electronic Structures and Photocatalytic Properties of AlN/Sc2CF2 and GaN/Sc2CF2 Heterostructures.

Heterostructure catalysts are highly anticipated in the field of photocatalytic water splitting. AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are proposed in this work, and the electronic structures were revealed with the first-principles method to explore their photocatalytic properties for water splitting. The results found that the thermodynamically stable AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are indirect semiconductors with reduced band gaps of 1.75 eV and 1.84 eV, respectively. These two heterostructures have been confirmed to have type-Ⅰ band alignments, with both VBM and CBM contributed to by the Sc2CF2 layer. AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures exhibit the potential for photocatalytic water splitting as their VBM and CBM stride over the redox potential of water. Gibbs free energy changes in HER occurring on AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are as low as -0.31 eV and -0.59 eV, respectively. The Gibbs free energy change in HER on the AlN (GaN) layer is much lower than that on the Sc2CF2 surface, owing to the stronger adsorption of H on AlN (GaN). The AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures possess significant improvements in absorption range and intensity compared to monolayered AlN, GaN, and Sc2CF2. In addition, the band gaps, edge positions, and absorption properties of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures can be effectively tuned with strains. All the results indicate that AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are suitable catalysts for photocatalytic water splitting.

Effect of the Twist Crystallinity of N‐Polar AlN Underlayer on the Electrical Properties of GaN/AlN Structures

Recently, N‐polar GaN/AlGaN/AlN high‐electron‐mobility transistors (HEMTs) have been demonstrated using molecular beam epitaxy (MBE). However, the MBE method is not suitable for mass production because of the need for ultrahigh vacuum, small chamber size, and long deposition time. This study fabricates N‐polar GaN‐ or AlGaN‐channel HEMTs on AlN with polarization directions opposite to those of Ga‐polar GaN‐based HEMTs without any cap layer and demonstrates field‐effect transistor static characteristics. In this article, the focus is on the crystal quality of the underlying layer and the effect of crystalline quality on electrical properties is investigated. The crystalline quality of the twist component of the N‐polar AlN layer is improved by combining the Al‐polar tiny‐pit layer and polarity inversion from Al‐ to N‐polar. Finally, it is shown that the crystalline quality of the twist component is a crucial factor for improving the electron mobility of the N‐polar GaN channel and simultaneously increasing IDS.

Contactless spectroscopy of photoactive centres in sapphire and nitride layered structures

Spectroscopy of photo-active centres in wide bandgap optical materials with absorption and emission bands in the range close to vacuum ultraviolet spectrum is complicated. In this work, the pulsed photo-ionization (PPI) transients, electron spin resonance (ESR) and proton induced luminescence spectroscopy techniques have been combined to evaluate the origin and parameters of deep centres in crystalline sapphire substrates. Additionally, the layered structures with MOCVD grown thin top layers of GaN, AlGaN and AlN on sapphire substrates were examined in order to clarify origin of emission bands related to the epilayers and sapphire. The sapphire assigned emission bands have been ascribed to the F+ centres, characterized by photo-activation energy of 4.8 eV. Prevailing of F+ centres has been confirmed by ESR spectroscopy results. The covering nitride layers of AlN, AlGaN and GaN led to appearance of the additional PPI spectral peaks ascribed to recombination on the interface between epitaxial material and sapphire substrate.

A Novel high-voltage DMG Fe-doped AlGaN/AlN/GaN HEMTs with sheet charge density model

In this work, two-dimensional electron gas of the dual material gate Fe doped AlGaN/AlN/GaN High Electron Mobility Transistors (HEMTs) device, sheet charge density model is demonstrated. The model is constructed by considering the electron charge in the barrier layer and the quasi triangular quantum well. This sheet charge density is calculated by Fermi-Dirac statistics by the formation of various sub energy bands and it is valid for all bias voltages. Higher carrier densities are produced and the introduction of the Fe-doped AlN layer reduces the electrons penetration to pass through the AlGaN barrier layer. The model's transcoductance variation is basically constant over an extensive variety of gate source voltages, thereby rendering it perfect regarding high-performance amplifications. The developed analytical model is demonstrated and the simulation results are validated with TCAD simulation. A good agreement is achieved with both analytical and simulation results for all of the voltages employed and device geometries.

Low-defect and stress-free AlN(0001) nanoprisms and microrods selectively grown on micro-patterned c-sapphire substrate by plasma-assisted molecular beam epitaxy

This paper describes different growth modes of AlN layers on micro-cone patterned c-sapphire substrates (μ-PSSs) using plasma-assisted molecular beam epitaxy. Ordered arrays of AlN nanoprisms and microrods were selectively grown on the tips of μ-PSS's microcones according to a bottom-up formation mechanism using sequential migration enhanced and metal-modulated epitaxy (MME) under metal-rich growth conditions at 820 °C. Transmission electron microscopy revealed structurally perfect AlN regions above the tips of the μ-PSSs, which initiate as inverted nanopyramids with {1011¯} side faces, evolving into hexagonal nanoprisms with orientations of {11¯00} and (0001) for side and top surfaces, respectively. The diameter and height of these ordered hexagonal nanoprisms, which have a 60% probability of nucleating, were about 1 μm. Long-term MME growth of these nanoprisms in both vertical and lateral directions led to the formation of AlN(0001) microrods with a maximum possible diameter of two micrometers and a height of up to 6 μm. Atomic force microscopy revealed a mixed step-flow and 2D nucleation growth mechanism for the flat tops of these AlN nanoprisms and microrods with an average surface roughness of 1–2 monolayers. Micro-Raman spectroscopy demonstrated narrow E2 (high) linewidths of 3.8 and 4.2 cm−1 for essentially stress-free AlN nanoprisms and microrods, respectively.

Direct high-temperature growth of GaN on Si using trimethylaluminum preflow enabling vertically-conducting heterostructures

In this work, we demonstrate that GaN can be directly grown at high temperature on Si(111) substrates by metalorganic CVD without using any intentional AlN buffer, by simply employing a trimethylaluminum (TMAl) preflow. We found that n-GaN layers directly grown on n-Si with a TMAl preflow not only present a better crystalline quality compared to the use of thin AlN buffers, but also exhibit orders-of-magnitude improvement in vertical current conduction between GaN and Si, thanks to the absence of highly resistive AlN layers. Our proposed technique opens a new pathway for the effective realization of fully-vertical GaN-on-Si devices.

Regulation of oxygen defects in AlGaN-based epilayers grown on high temperature annealed AlN template

The regulation of oxygen (O) point defects in AlGaN-based epilayers grown on high-temperature annealed AlN (HTA-AlN) template by metal–organic chemical vapor deposition (MOCVD) is systematically investigated in this work. The concentration of O impurity in AlN epilayers is restricted to 1 × 1017 cm−3, but decreases by 59 % and 27 % when the AlN layer is replaced by the Al0.5Ga0.5N interlayer (IL) and AlN/Al0.5Ga0.5N superlattice (SL), respectively. By analyzing the growth conditions, the O impurity can be inhibited by raising V/III ratio or decreasing Al component, which increases the formation energy of O point defects according to first-principles calculation. This work provides accurate guidance for regulating O point defects in AlGaN-based materials.

Controllable regulation of diamond nucleation and growth on GaN substrates through pulse bias duty ratio

GaN/AlN/diamond composite structures were prepared by the pulsed-DC bias-enhance nucleation (BEN) technique. Smooth diamond interface on GaN surface obtained for the first time by BEN technique. Different bias duty ratios (30%–70 %) can regulate the nucleation of diamond. The nucleation density increases and then decreases with the increase of duty ratio. The highest nucleation density was 1011 cm−2 which enough to grow a dense diamond layer on GaN. The effect of BEN treatment on the interface was investigated and some positive and negative results were obtained. After diamond deposition, GaN layer was protected by amorphous carbon and dielectric layers from hydrogen plasma, which implies that the BEN technique has the potential to be applied to grow diamond on GaN surfaces. A smooth and complete interfacial structure was obtained after diamond deposition. After the BEN treatment, 10 nm AlN layer was carbonized into an amorphous structure. BEN duty cycle can modulate GaN/AlN/diamond interface structure, which may reduce interfacial thermal conductivity. These results demonstrate the potential of BEN technique for diamond deposition on GaN substrates and provides a reference for further optimization.

Perpendicular magnetic anisotropy of Pd/Co2MnSi/AlN/Pd multilayer films

In this paper, the perpendicular magnetic anisotropy（PMA）of Pd/Co2MnSi (CMS)/AlN/Pd multilayer film is studied, which is designed based on the two-interface interaction, the orbital hybridization effect of Pd5d-Co3d electrons at the Pd/CMS interface and the orbital hybridization effect of Co3d-N2p electrons at the CMS/AlN interface. Compared with the Pd/CMS/Pd multilayer film, the PMA of the Pd/CMS/AlN/Pd structure shows significant enhancement with the insertion of the AlN layer. The Hall resistance (RHall) of the Pd/CMS/AlN/Pd structure reached 0.02 Ω (increased by 300 %) and the coercive force (Hc) is 388 Oe (increased by about 76 %). The magnetic anisotropic energy density (Keff) is 0.61Merg/cm3 (increased by about 135 %). The PMA was optimized with the film factor (layer thickness) of Pd(6 nm)/CMS(5 nm)/AlN(6 nm)/Pd(6 nm). The X-Ray Diffraction（XRD）and High Resolution Transmission Electron Microscope（HRTEM）analysis showed that the PMA is affected by the crystallinity and interface quality of multilayer films.

Single-structure 3-axis Lorentz force magnetometer based on an AlN-on-Si MEMS resonator

This work presents a single-structure 3-axis Lorentz force magnetometer (LFM) based on an AlN-on-Si MEMS resonator. The operation of the proposed LFM relies on the flexible manipulation of applied excitation currents in different directions and frequencies, enabling the effective actuation of two mechanical vibration modes in a single device for magnetic field measurements in three axes. Specifically, the excited out-of-plane drum-like mode at 277 kHz is used for measuring the x- and y-axis magnetic fields, while the in-plane square-extensional mode at 5.4 MHz is used for measuring the z-axis magnetic field. The different configurations of applied excitation currents ensure good cross-interference immunity among the three axes. Compared to conventional capacitive LFMs, the proposed piezoelectric LFM utilizes strong electromechanical coupling from the AlN layer, which allows it to operate at ambient pressure with a high sensitivity. To understand and analyze the measured results, a novel equivalent circuit model for the proposed LFM is also reported in this work, which serves to separate the effect of Lorentz force from the unwanted capacitive feedthrough. The demonstrated 3-axis LFM exhibits measured magnetic responsivities of 1.74 ppm/mT, 1.83 ppm/mT and 6.75 ppm/mT in the x-, y- and z-axes, respectively, which are comparable to their capacitive counterparts.

Reststrahlen band infrared damping, microwave transparent AlN/polymeric film filters

The low-temperature (t ≈ 20–30 °C) technology of hybrid helicon-arc ion-plasma deposition to grow the amorphous AlN layers on polymeric substrates for getting the IR blocking filters was used. Flexible substrates based on Mylar, Polyethylene Terephthalate/BaSO4 filling and Teflon polymeric films were applied to grow the AlN layers having the Reststrahlen Band at λ∼11–17 μm for filters, which are damping the IR radiation in this spectral region. The Polyvinyl chloride polymeric films without coatings and the Mylar/AlN structures were used to make composed damping filters for spectral range λ∼7.5–25 μm. The transparency of such composed filters in the spectral range λ ∼7.5–25 μm can suppress about 96–98 % of radiation from the objects being at t ∼310 K. In this spectral range, the objects radiate ∼75 % of the total radiated power from a black body in the full EM band spectra. An explanation of the behavior's peculiarities of the spectral dependences of the IR reflectance of polymeric films/AlN structures with using the classical dielectric function model was proposed. The transparency of all polymeric/AlN structures in the microwave frequency range (ν = 60–270 GHz) gains T ≥ 80–90 %.

Diffusion Nitride Surface Layers on Aluminum Substrates Produced by Hybrid Method Using Gas Nitriding

While gas nitriding of steel is currently used in industry, nitriding of aluminum alloys remains an open challenge. The main obstacle is aluminum’s high susceptibility to passivation. The oxide film provides an effective barrier to nitrogen diffusion. Attempts to overcome this problem have mainly focused on glow discharge nitriding using cathode sputtering of an oxide layer. The produced AlN layers exhibit no diffusion zone and show limited performance properties. In this work, the effect of hybrid treatment aimed at producing diffusion layers of nitrides other than AlN on aluminum alloys was investigated on the model system of iron nitride–aluminum substrate. Hybrid treatment combines an electrochemical process involving the removal of the aluminum oxide layer from the substrate, its subsequent iron plating, and a further gas nitriding in high-purity ammonia. The obtained results prove that the hybrid treatment allows the production, at 530 °C/10 h, of diffusion layers of Fe3N iron nitrides on aluminum substrates with a nitrogen diffusion zone range in aluminum of ca. 12 µm. In alloys containing magnesium, its unfavorable effect on the nitrogen diffusion and the functional properties of the layers was observed. An interesting direction for further research is hybrid treatment of precipitation-hardened alloys without magnesium.

Van der Waals epitaxial AlGaN/GaN growth on hexagonal BN via two-dimensional N-induced dislocation slip

We demonstrate the epitaxial growth of nitride heterojunction structures on two-dimensional materials. The growth behavior of Al atoms on hexagonal boron nitride (h-BN) involves preferential adsorption and diffusion along the N-top position, considerably improving AlN formation in the early stage of nucleation. Preferential N adsorption enables h-BN to provide a natural N surface for the substrate. Moreover, the natural N surface provided by h-BN can induce three-dimensional island-like order merging of the upper AlN layer, promoting dislocation slip annihilation. The dislocations annihilation depth of GaN films was shortened to 870 nm, resulting in an order of magnitude decrease in the density of screw dislocations to 5.50 × 107 cm−2 and considerable improvement in the performance of HEMT devices on SiC. Therefore, h-BN films can be used as two-dimensional insertion layers to obtain high-quality van der Waals epitaxial nitride films in GaN-based HEMT devices.